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: '8e9c79115b6d9e32',cH: '5w04YMfRGJghTQRRQ_Rl9.7YBDxabdmuEP1QgAQQxe8-1732819085-1.2.1.1-FHrj61b6Nn379u2llBfy40apZTn0vQPLd6uUUqCLfZ6Zofq_E3hAWL37RAnJffx3',cUPMDTk: "\/doi\/10.1126\/science.276.5310.266?__cf_chl_tk=MaMzz0J_oJvTY79ngAEkf48rNMfTX282oErDb5zott8-1732819085-1.0.1.1-tvj.yhiRNYrjMpa0pPheDuC1V5SisHta9AY5g_Miyvw",cFPWv: 'b',cITimeS: '1732819085',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/doi\/10.1126\/science.276.5310.266?__cf_chl_f_tk=MaMzz0J_oJvTY79ngAEkf48rNMfTX282oErDb5zott8-1732819085-1.0.1.1-tvj.yhiRNYrjMpa0pPheDuC1V5SisHta9AY5g_Miyvw",md: "N9ioDK1d7pcvFwZnY6IZJFBYhF96sYyyAjOf7O03UFI-1732819085-1.2.1.1-jQLl2n8j6zFpUfj3p1_Wwf6KCUDjUNn4Z863qCmvBdmxUNI_NyprqVAFAWmzZ4IreiSqxw.HFMP5T_GGzewAl4UPW8byICAVVYeKL1._eLM5KmWWkOfD8CLk41UDdfsEpSfqNOFZ1.ALNKC6jBWNYYGZ5IHt9rauoxDbKlN0k2L3ueOQs2bFZqJIAxnoCHvCWnWRu_XVoP5YbubRKiF5eBJouEDm6ZzipGQlv09KQm0SYXoEaywV8cuEIgzoDHPlirMTMg.1WHtmkXTYc8cXh0W4Wwn6rRekryeSdcXiTQvbnEPIoc_D7bcTulMzNHw6TxnuM3zKe948dnTAOslIT203J1Ff1cXKHt4e7b3DHvhVJKme1jFFRLsc2BGlRiIgTa5EYBit.Eni3tpIl0HhVSnVBFSwjyQxDaxL4uHcdjQjHN0oSR2tAhldWtTMVktVlzUu68poFhKrrmkapaLcAcE8giQHIjY_gWYcVmmzRmxRsSQWQPQmfXML8g.6IYUGQfBgxfm.q.dWk.exHQdkIUsnsE11s00UJsJKDeWDS0PKTi6ZUPfN77rUPOgVTaMH68sjrSjNbu_GITVVqG30WjpWpPuAb2VNlN.C7Rw9Njl7IZt3uGhFUmyY4XPpOK7ui3G_VQDdLqaXTIsMy1vPb4_zPmx8Hmj7KSxjRdJl56uLW4Rb1ywGA5W4FO8B5ADSIaAGX9704haeoP3zbWnSa2uUcXDAfHR3_DO6HXHvsQRcaXbC1sg3hIwyJ_eZQRVHDeSBBqshhluLR1T2O2ToE5Iiow4g8TBwToA6LHU7qSbgXaxrnXanqI.v3Of3FObrBF3KeSaV287OMLi7KESrvgf.19aoDS9lNu4RhjpoHSPex.dk.JqfcFNOYGuBHJyetE0CC_qMHpd4wgN9pZio6lsXcvC25ObsTIrRkABfb6OOzAL0KVHyYSIBx4f8nuLCBDAanEcSMmE90W11JdpaS50aZ5gUo8WgizcgdPVO6QB.Kdozp57oTNuHm6l7FACHduv13GE9dqIHIuIrlSKGgE6rmRjy9MvrWPP5l8_Y_WQJNNp.dLo79WCkVGhX9WEPzVGq3T4AjtlU6_m6TH3fsCjEdhNjteX08HypfA3Ni.rFiMKfRqSczl4hQfdiPLo7625ZjdzaYTCHPtDQc9iUTtmSD4_EG7cWZPSVsPC2bWBy5b_2RP3NY4hsB76yFgA2XASv.Wa38x7emOBKqvQElor8rcdkWvyNudhY3hPFYXLCFDT5D02pMexEip6iq1JcK_OKEkokBJScM2UJumnNjhHLQ9OOrYycC4r26o0LTxtaYwRA8o444ElDqRWriVX6ViaDud9iVRnOGOcl2vhCyJ4N0Odih4c_elZJW4o8mS_3AL1BG3GI3Am9wgp6TdwsopRNz5XTs9DF.OEAbMHOdanb2p8my5JgSHSGPuNnYt7dWixqlWxgdABmKMwUbwO38CN3r696YYm75EvuzZkq85fwEXW5oKuc9F4hxLtqB8qrb9KscBUtjW3AfUASoNZ7sR2vzgVnlQde7uPwy39ILxzQZ.QjJqozVrFh6xg_9paIP_XEai1Ty21xNIH3V9sDONvuLjqqrnDrJ4D_yn6RYg2_M7SCI8SThCdswlIzY8o0frlns0sJYODKVKOCQS32SBxK3vsHSDN8XJSyDiPHju7NjYetrGIAAHrSLTsUe_Nd_qdAMzCQcEGi7zutn4oKIK95MRTPXzt4REbHkjZsR9gb4F.YBAa4tJN7GBWMUvvdcgJnW_Eov1dodrXaoT90Z_6AppaVzYQnZTQVaZAr859d9rB37BFhjzQ8KzN1pF0bFYtlbzLwdxL1UkTB_T7Jj0a9hH5ThB_X8W4cuJGlZd.TilJyojl9BbO84FIP8kDzSuuI7CW8f1ylsrqWAah_1qe18DTRcUOGGhp_EUg.Lzd1bdVWbH460QpHTziaR_ygk8gNE8JgxP7koHCBMRjWkIuSbERillLd5dm7oEPdxggm05Dc3TuGjUB3.MyKJ19NJo9w6xR5olkuUZ7U_5lNkbZeQX90tTyG24hg4YIW4NE5U.msi.42r1u4Wa64nkKwQjzC4OJotp5YnRUgAlQXey.HkVuSvO5inKQge6dfwhFLjwWI6l4ivEjqcsJcDQe.V52XmNu1xrX_nE2CyIrJWwI9_X27kJq4s7FAw1sXy.hHgZukw8YCzJktIO_CLcK9Ym3Iem7g4v3hCeOI7oO_5QTCKx.wRnUD.caFBhSH0YP_2g9Eye4FLgeiEzpoCNkFmEj1Y56iwTgHRIW_n2o6Nc5UEfPHgybyO49VMM3VZOfwXV93.1LM_OPpYEvUnuPIgCr94UYND_Ay8tZVntoeEldRBLZnJf1SiWShCRuLfWUAEwGd0TU6r._r3ArzAMHw2Eii8FYCqRfpsAcLkC4bXOYUUFuVn5MOj7KioioODLAnj1QP0LBwBR8cU8Ll0MXu8T700WfDHe_KDQq6xXNJ114180gpzNGIqJCL_._Za3Gv_pVR0pdpfGHGCgk9utD1lNqmmZtdsrymw8DF6I50CUfvPRtnuLQSlPs3wyG85fIJZQQKgAfdQyRBlbY_NGx9F_dD5ziAgzj_WdKAnrnmWNVaxgKbiDJa2ZUwRgi6GkYCCZbXHvl4DWgQ6M5VWrHgjTVJMV7hoNiLJKAejA6hxOxACL2ka78i2jcSyWkf8mecIK9ApRqvSLUVOTT2Ta8XZCxdXZEIaxAc2mpKOfsmS6cOis80qftIFu9nnY5lJ27pFGKF1tr85GMPpMI39PAGjYFnbgtBZzs9kMsf30T2I44NHoXPm7XmjC4sbTiFvUlK3l3VdofB6QkB.llICUi2Jqh2CzeTwy_hNV9.336BjUhmBU6B.YJNSFqftKkYul1W5kcay3D9rY5MUL8mzjJA1Qe2ae5KhjyBawr3CIVCvvgOALdgpvolP6a33Ia8TLuWstrI21VTAEAj9PrvVJYJJn8Z_5BRzDyhnnuXGGWS",mdrd: "yab95m3xHN6hWQfVA7J.K41t4HxA.0gVqzJkHwFRe54-1732819085-1.2.1.1-xoeRXp0cDscNPuY.8XwtaoMxenOKjeR6oZf2EliHY3nXziATsrT5.jT3rpO4Qy0aF809aY0O9Ugwl0XW4bYbXOy_2PGXpIj.yP20wurbg_oNz76M2nVsoQBL3mCfb8HP6XAu.G8MVBtNrK1yQ6NBYqWBSpNRWXcfRHv6rgEyKixFvds9wMKZ.C_uwWgnWYZPf19lRA01XDL_4AKMypEAZqvi5.PCkXtwHD9TY5Wm0ezSIzX0p1oDRUsD5LjbkUbLDOZm3oMhW0kytSGkObbwuMT7TkbvAOCWRBvppL1E1M4UVtSiEiIO2nKFR8E3wgc_KoNr2xM6CsvYc6g9gZMBE..qPbPR2951mhn6RqOQGwn6RRqSNHl6l325eszGazVRne7mv9BNGTSmauHVYqMbRysu3DUImbHGHzvkkPR4zaOOMB0sRACDm8pgy4GR3PaUZQu0HsqELGLlGSWVdGR0Mk8JpnX5K6xlK8mQ.M3ug2fawW_zADNYcDF_lT6XG3d5_W5P_hFyE6_eGWBbLXpk.1MWbynz3qBkCHtmAOCPCGBOE8NDT.vbBH4jjL7sdrSOMpreUEm71AD.jDGjhtcOUia3e1Ir0I6Uk23LoU_N02jBiY9kcfv89qLnCqpqjbpaRiTW1Ue4NlULjGqc2bKILFMBNjAMnSY8WQcx86SbN6c_KBqPiXAfSTOilaUNIHab25edIlJu4LEnsm8t3TLt.JT.DqNfy3zXiSOUtK.SLdJyikAO56lF_va9mMKMC6EjYRu84Gh.DL8MIkzeE9q9jKc__rbdkZhsi_MzaBPZn.vMSLDh1XgMrJpS9REf_EMkX3D30xlXgsbnT1wGTtsESBGCUoMC.a9oatpLF51JePEHEK2IDXqi3.Mus4Y9pBfb1NcrFMKkXlMCQ97Uj84qhYE.Gzot96QHRVdp57HhdmpgC2vUeFe2rgRt2VrbNL1FXoVoznEfqsFQDEwBA3IpNbh2Vy.g2U4UKin_VrAFmuyfDvgyACCHd83o4nQlbgRSpf20b5Q1IGM_sDp8p4HoewTSSKjh1V7PY4SoAYN3jDKA6P7Y7tjwIyzN2fCuR4CVGGhffJ5UXcinlsYDkfOxQlLsh_WuX_.AY5HpfuDj_jQxmStETu0iFXf5WHSqubF4lX76k2kWa1AKiMuJB73ZeI1BC4WO6rSLKk6pfizoNwoADuFCo4XSQe65mbC.xy9uyp76HfItzDTk4OZsoKC0XXKX1XMNO.s7Mx1PgVrILmJtdTUhl0BghiaZRa5XKkmTZnf541XMrpjnWKNXkYzT9PR_j6WD_e_fv3rGvy7hOlECbjxzVMC4lwqceTsW_vg3TrZjCH8Db_ABM9a3ip03HiSI.JXyllD73VPbMcJ5j0_kjU7YKqQi.R_e0wzN6tp0Aui1OCSOvBPV9nh5tGlPd4POasMceYdVndxIssLTDS79oPACh8IjwRXSBAt.H.PmbwjvZZr5NVYHt5sPQXkd5Io_q4H8Uo6CfyKT_BzXzJqDYPkVPlme3YS_jQi5vspfLPdyKGYglcxSvSrrcQJmJSaPLdAoipJs631CrBxsC8IZEydtGxfzUXUGJGtFh7N4mIH4VNiD9w_uF9zZuMYhl351388jGX7HphiFvgxSKAe6WufDQA0ZNkkfGB_Jyx3TvrKl0_f9yrouHIYJgWGmjEaq5Z6V0k5zwV4wDs8AQNId1_6Y2LV8AzqxA2KO7e3kMDdMkoc.zu6Y44FYVTB_I7UWWFhHAHBz9qaeUZd_3fZp3m9HUqAZ0wtEYNZMkAdobFzTaJBuHfPHt9bCVGe6GowipeGA2XYKafaZoO6ehfVPVlLnE8h4Z51lT0J3AEXJYTy84T7.kSP0KHI.T3iioB_f2JrtnXS6QmQDeOUeuYTtuHVxPcSyvslegN17S1KQNyGw6RQczk_zTgetRhqdJaBinjJfJC.5Z2F6HfnLoMM_WNpTKT7A41kQ7cQ1BD2r"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/b/orchestrate/chl_page/v1?ray=8e9c79115b6d9e32';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.276.5310.266?__cf_chl_rt_tk=MaMzz0J_oJvTY79ngAEkf48rNMfTX282oErDb5zott8-1732819085-1.0.1.1-tvj.yhiRNYrjMpa0pPheDuC1V5SisHta9AY5g_Miyvw" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>