CINXE.COM

<!DOCTYPE html><html lang="en-US"><head><title>Just a moment...</title><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=Edge"><meta name="robots" content="noindex,nofollow"><meta name="viewport" content="width=device-width,initial-scale=1"><style>*{box-sizing:border-box;margin:0;padding:0}html{line-height:1.15;-webkit-text-size-adjust:100%;color:#313131;font-family:system-ui,-apple-system,BlinkMacSystemFont,Segoe UI,Roboto,Helvetica Neue,Arial,Noto Sans,sans-serif,Apple Color Emoji,Segoe UI Emoji,Segoe UI Symbol,Noto Color Emoji}body{display:flex;flex-direction:column;height:100vh;min-height:100vh}.main-content{margin:8rem auto;max-width:60rem;padding-left:1.5rem}@media (width <= 720px){.main-content{margin-top:4rem}}.h2{font-size:1.5rem;font-weight:500;line-height:2.25rem}@media (width <= 720px){.h2{font-size:1.25rem;line-height:1.5rem}}#challenge-error-text{background-image:url(data:image/svg+xml;base64,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);background-repeat:no-repeat;background-size:contain;padding-left:34px}@media (prefers-color-scheme:dark){body{background-color:#222;color:#d9d9d9}}</style><meta http-equiv="refresh" content="390"></head><body class="no-js"><div class="main-wrapper" role="main"><div class="main-content"><noscript><div class="h2"><span id="challenge-error-text">Enable JavaScript and cookies to continue</span></div></noscript></div></div><script>(function(){window._cf_chl_opt={cvId: '3',cZone: "www.science.org",cType: 'managed',cRay: '8e81199abc2c4810',cH: 'w_qiyn9T6Zf1a8W.gx2nl7jZDQboHXH9UOzPzUYV8og-1732532059-1.2.1.1-9nsICYsI5r7aszQom3q5npxrDKLOMev95LdwP0eZOAZe9ri0onyWZQEtWTHHKmxQ',cUPMDTk: "\/doi\/10.1126\/science.249.4970.735?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub 0pubmed&__cf_chl_tk=ZnI0LxqoH4NQfR2CA3rgice.ww5ztTOGDWCEZ4Jb0uc-1732532059-1.0.1.1-43JIMa0OCWMi8rb3t8If8_diLR9TNDjc91ANaHJdFkk",cFPWv: 'g',cITimeS: '1732532059',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/doi\/10.1126\/science.249.4970.735?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub 0pubmed&__cf_chl_f_tk=ZnI0LxqoH4NQfR2CA3rgice.ww5ztTOGDWCEZ4Jb0uc-1732532059-1.0.1.1-43JIMa0OCWMi8rb3t8If8_diLR9TNDjc91ANaHJdFkk",md: "tFnKQLNFRgxLhaCtXbSjs3etAfkiHWBkRrcke1dTZsw-1732532059-1.2.1.1-5O90sFKy3BWi7m9RV7hjlvnZaTJAWIBM7LySBeNWpqU8vAPbKjdeo2BVWCzVJNuc5lAL.6eSszIxKJnEoJ12qpYE0iwpUaNb8K9ValBAUTu6m4uhiDDkjGrU4ZbIJGCygTi8Vhfyee9Hk01voeEETg54SjS3W6bc1CwYEnLax2w1EMNXlVFJeA9AtyTqTbxo2c3LaFHf0h0hkrT4uIkYJ2YK9fmYURUGKf9EyWBcyU5OeUrCMGVC3x3LyePOKhrG9KmUiX4epH2Ghq91UXM4sBl5Af95xzQkkP4LX8HsF8fyeTq.MfV0xH3DME6xNxKw9KuQcP14V_aOmj2tSlrRjFpahLeFhvK_Q2QABvn2jMWSQuKa_OLh4BxMgva5QNL6n7Ug5UdUzLzGfjVzgFPok1kKPwz19jDJ5LXvP1aCvwvuo8xq_x6lC7R.EByU.MKkYIYy8.BGE1urZrfPCNjtKkn9MwgAflC5HF3dV.Tj7oCLPf1JpqjBcLRElqSYda2QhLY9KZE0zW_CE6M4JZS.H0tzHIlw3Ft19ia62BR9wZOcfaSGESU2gM16Im8ZUa9Mqh5tjuHiH5fPuzMydHh08nnFBQ1kVIvL0DpGgi4AAwh8SDjRf1YRKhk3hz9nHI5yKKiTdyNT.gVryENIXbIlnZILeUlRB007wIdu1xdqKSA0xlP.x4MD.SH_0QkuI.wwk5aKqrchyjyVNAQu8J1gT.qpXnCWpeRDqA772cVFsoqIn_xwFdqcOGZJwgLoQqXvBEfHpTttABiwwjFfkNDkEZz1o6DeSCytXddVrqnCEu8TzmmaqPwd.bmblOl2BhDQrxdZUWQn6igtLW.nV0ZvhuQh1FWakF18bszCB9ERw5OeWl4SeudZlOhNEhp8DQ.vpFx7Jq5FYomWY0yXO_BxTkVIt5RA5Isdd_2HUR3ecSrs9DIUtQHH9xgspb9DAUZv8wtCVfiR8P44._28qvNRwMuUJAM30xVrq7v9TqD0DwCj3VespfyNMM3ZalWsoGJYNi891g35H8jBbtK4_9fXMsX15X01VdmMVrohOWQoxhHGSpe8is7YS5J1ZL.blKqySrZu6dWFUQ3v8lezy7xUw3bG8ifUGKBvXR.bflepuXKydnpGX_vB4H26t8.AN35w2sYIUp8rKDGRfABDlpYhdTebHhdq8RKZ0UNKxi25_dUngvAUauS_S7g90dttoTcZfhTzHtRGXkx9Z2yJnQ8zS9SZ23J5E_W2cqZsVM5Y8C1A9ZWc_0aosms_IgeezSBX4ACFqKfP.QAejn55yMAhDB5Xa9b9XwNV5NJ7od50Z57VbeYBYT945GSeK0nUwOZfzbL8KlhvsZ.l2hFz_L503PfU4E_l72WijJhDxBvpmYjL0vMPKpDyydwbcD_KPqLCyOlh3CagG9VnIat.G5PhuerhIHZgeYvbalw_p557KMcFlBgyf3yyQv27t7qAIQpR52YO8oPLbq8oXD9KqrKOiIbUsgtF5D890WdiApqerf97i2s5IgIDfXOZsFeS4dnAiox5zK.IRrnrr5Nx2P26xh6MwuMLVwm7u.lTh8nRUMZhp.tktFP1bmbPl5KkZTcxjmg2t4eUylzljAJPJtGguQkH1I0Ed2b5qYrW2QD.npEgr3aO5I7SZ01h0KeZGlgqgc9xClE8b7I1OJdWcZvVmQPq_9rI5KG9kaKYS0eCeU_Z52S.bH8haxhi1pB6SMtcgKwUqvxo4wj1LwpmTujIenTMEAAFNGkPj6qmuijTbN1UQvV5GgSDym2HCq5rCW32KUvuXo6JMJeuJ27C_4Dgzjn2xYedqIoQaSMyG6lbf8AK9VcI4BYuMuaf7jQmltG2B06S2_HhxvHpvmhNMc6JIR4CcIfoanVmmi12sf.6G4xE3KDzVTc526N6oPAH5JdbTB9V4YM0rPk9vdpEJ6ICcdlSifiL_3FUBQo2hQJVd7dpDuIXrnpy3hKSu_IZ_NwmMc4rj3X8IJHm1ZGAJ4JoJLyakr25vp1_2exohb2cVxZvpQiJjPvGx2ykuMlBMwkkW4WGScr9PSt76y5ZMUpRopxLwPXgXS24ISFaqm9I79RNbzkudeRVZljd0RrRkKBVeytk5WU0u21qfUgLilHtopXPUWUQV.GyzFogA3F5rGqNly4wqOU9e81LkuxQv797jtoolXoXkU0VpxepHMXyYpMPc4LQSHC9v9k6DUHveQuiSzp92GZcky73KOaVMxQw6gSfQRG_IWWu8GgEZDr7X3AAE0DYBQ7si4yotj5J9PfQpb0iJqXkYzu6_iPG5QWh.Xt4Nq1jg0LFauJ3sFovhOanFPZPQ7tDyJlvXQnu8ddxL5Lk6mItDvUDoHjyg_MV8NigIZl.KZOnN9N5HdcN_k2XDULbX0Ab8ZvUzBHvL93iQgnUaw4Au.ynAk3_FOsKv9o53w4R7.TGq1ye0YuZ1FeTzYEUlv9eNf06LhKCOf_tu28ZpV9N6OmRw431Bb9xg.qEVreRxBo_nNvA0AtYYlP1uLqGMH3TTZGebrfVEGgC3zMra5hCsmUxuyUq8gblzeiQbQH.Ce3sFLLFbzOMC.lvcVAx.nNB4lYEzrEGKwrtd.QPllO3uUiDPqBZn.1T7LOBu4li6sC1EXoQNuOH8haH6NieaYVrFBHNCJHZezeno_1rIgSB6YlxEJ_IYOOKkJDwGSv7yYS9fBM3ytJnM9iHSYjzkIlMPOH8klGUQLKhR369lgg0hblKItKmOYtwSVr7luGD4sWgdlqWTKEjva5LF7mhR7AUT4rb2Y6N2Qz6l6EG.jnrscrhYVwBBGsa5qUs1kA6rWWSzZrHKHvv66hdqe2LGE8XyiC.rDUuvATjF_UzYTtt.dUBEgCWQwQcTn4cGNk8pUsJoiqyQut8QGEAqw6IucgVVDB.ARHqZ4asjz4GHHcm_JmyQ2JD92bGY2XzSUTf12VBiTKh.FwSz_wf_gvXHfYoD5IDDSY2RixQ9mULkK1oBy1jauVbaHGxTCUOfYHN5VYkLuKNqzmkqT4owb7MzCGToCcjHQdW2rcGrwKXyLq2t3yTiHyHHCGeJI0Aquggz0JUPxysOg5L8X30uyaKBA_FSkO_.FhgPhzZsTJQNnAczJHLeWHthrF7DK_9mrzfnruwU3on.GcP7e8qdo_HZDq95PERh1ohDgTrIQ5SXzOjlehwn8p0xm6G",mdrd: "pWijL9SWc.H.VZexbGRqk.6oBYgzKjT_a6qXt2BlmWU-1732532059-1.2.1.1-2FSDdKIE0R2i1NQwbTHwyn649Yw.UvU3pQHSAYgLqTa_B9vIuQUm34rCUC7e0_.yMPDJ704jNCliATugh8j2bvtEMBRB_dhX401Kb0EqdZsNlj8pr_Iz_mojT3tvIlL6VUQ9F4DT48snkuxaO1L5Xlqg.1bdBl2nvLh0pRPy.n3oLA1aHx4TrO.Fzl0uq0GzsLyve8cT1X21hG_85d6FwQCPE931kQE7OKEuQ3ZgWqbDjQ2zgMzpb36vHjcxJLUDnbBIN3WD20q217GhHHU.S8mRFV5OqymVKwY0plEY5WuK8cyJQdcz3f2U9XtZlRFwVSvpzau7fWXDPm.3lkUw5KB3u697w8KuiRYDu7DaiW1xWTjQoHqDg90Wa7UKwKalC3T1Uu9jcq8CCJajEDKRwty3EHEuUhA2qBkR2upOpL8hhurLTuO6n8FF.TtBkTTHgs0J8oNwwEBH9JnIQ9elZe1WvHIfBmwcacBymccRSe0RiMLcOUJfonEL0R1tBk4Eo5pUodyO52a1xuqcCRps0n63Jgj1wL4mDla2Hk4YYyKsjvir_pxJ4DbnEvFG.Xexy26vnZi9sWGcY8wNVKiU2VImuhv2rlwrIN90fQ6463xeQAA20AxDji3CF3dPZqBArBjrTZUiHjy6wpNXZzkiWZZxtqqnstxPbKKtZf.dPsJwOJK1Rx2TZdq0q_slOdRIk8Q3Az.EpfR1UjrPEEt_DHCzeVsJ_ux1ffXXzFaMTjvr4wgVY0NJXApECzUbcOZmtYhJuTo4r1xqzmwYn5X98ys_RsQ89WQ7pM.Lx41IrIPnPyMVexQ_2eZnSHfvtTpPzgZa1OMB6Ee7xjUG._o2ke4ihBRIAKIs4IPjveITwWCUVbOCDr7432xsVjm1zLaQ6xA5PQysKYHFjLcQ4k19qIlFT41z.qi2JjMczX_XV4RJrd_RYdgyAwVB0Cjsh8H2Q3n3iaMSXB3FObD5brYYpfHbufzbBmA91kImK4mSepGoIKopyz5i6ZPHZipXi3p5D2n30xnxJhWqkgg2.1rHavufpqvB0iNoTBAQ4JTssMzb2LspPZyqN5n38NN_9WF3zEaIMU7fLSvD1nDtQbtJBjB5QS1lABLbIT3c.4fthNAM5_H_RiXfSVNcW2C1h.arVNuKwCBOAotkWWK1J9HKmh2Y5KvibuPQg.U4S5Gxa0RoJ37bMMO6uuUm3jXpFbYxcvFT8Kf5Plhk5t6zh_LTws.Kgk1mugtNFIy9dyBq7RM6RhVPHertZhh.H7AsDwAaNnUAYAii5sZbp3kw3U5_BQryAUCWN4nZvzbu7.Pak5GFwk__k1UFUgRNNHibfKOS6bqNud8D0RswZrMPoDCty5UdWSxJ68Y1YQNCZ2.f22Y.DRzvvLFDopHGrKitTO7_Hm1tz02.kYdZyvG8R.mZuALxsG9qYOU5CYvxz15CCFsq1YzLsv9H0pOfcDO6R.zjHsBsskkMndplY6Kd82fHIoFo1T8dEc79TCQbKGLG79qXj409jRZxyMMZGAQ1JcEOgYnwszxJat4g_ECDHRvjTnUYPOspKcxlT7HGfPcIxAj8xxC3e2p_4x3uTtq6d8lkyGLg6AaIOY0S9vvg7RkQ_KQ8xTkAkrAmyEsFvGVUtvG.3tF_H_OBcxybb4OO5JdN5AT3EqnsX2J7UFrmrHOfZWZktEz37GHLtwRymI9h_8FmAYCoHezuc2teXU0Xpkxruj0RwTkSwnNuywg8oKHbvIgV9ALtebtPuDe6whfA5VX1u_KuilIcMzOGQ.pJD_U_JGQbuvhn5iXodItVm_rdc9daiWa4KRZeyhcUMH5equi_I49_lhSIjpIVuszdB1PnIQyTWed93pQCwNnTDmkKfY7o_f8d7dPKWUQGYBwhhhze9__KzFTlSEdIGeoxDXgylVSEnGF1zMEsUTwFJk44JxmvgRA7CTewIQYoAm_UZjs"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/g/orchestrate/chl_page/v1?ray=8e81199abc2c4810';window._cf_chl_opt.cOgUHash = location.hash === '' && location.href.indexOf('#') !== -1 ? '#' : location.hash;window._cf_chl_opt.cOgUQuery = location.search === '' && location.href.slice(0, location.href.length - window._cf_chl_opt.cOgUHash.length).indexOf('?') !== -1 ? '?' : location.search;if (window.history && window.history.replaceState) {var ogU = location.pathname + window._cf_chl_opt.cOgUQuery + window._cf_chl_opt.cOgUHash;history.replaceState(null, null, "\/doi\/10.1126\/science.249.4970.735?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub 0pubmed&__cf_chl_rt_tk=ZnI0LxqoH4NQfR2CA3rgice.ww5ztTOGDWCEZ4Jb0uc-1732532059-1.0.1.1-43JIMa0OCWMi8rb3t8If8_diLR9TNDjc91ANaHJdFkk" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>