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,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIzMiIgaGVpZ2h0PSIzMiIgZmlsbD0ibm9uZSI+PHBhdGggZmlsbD0iI0IyMEYwMyIgZD0iTTE2IDNhMTMgMTMgMCAxIDAgMTMgMTNBMTMuMDE1IDEzLjAxNSAwIDAgMCAxNiAzbTAgMjRhMTEgMTEgMCAxIDEgMTEtMTEgMTEuMDEgMTEuMDEgMCAwIDEtMTEgMTEiLz48cGF0aCBmaWxsPSIjQjIwRjAzIiBkPSJNMTcuMDM4IDE4LjYxNUgxNC44N0wxNC41NjMgOS41aDIuNzgzem0tMS4wODQgMS40MjdxLjY2IDAgMS4wNTcuMzg4LjQwNy4zODkuNDA3Ljk5NCAwIC41OTYtLjQwNy45ODQtLjM5Ny4zOS0xLjA1Ny4zODktLjY1IDAtMS4wNTYtLjM4OS0uMzk4LS4zODktLjM5OC0uOTg0IDAtLjU5Ny4zOTgtLjk4NS40MDYtLjM5NyAxLjA1Ni0uMzk3Ii8+PC9zdmc+);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: "pnas.org",cType: 'managed',cRay: '92337a251860919c',cH: '.0B5K__BaBphDRfKZWy.POTpOA8utfqH.rmWVQKjZPI-1742455542-1.2.1.1-4PewufAz83ObXFo.ZPJ6HdFNmjCOHQ05YiQfZ4_uOUTx1FVwhaYT9ybfQRrpwl6Y',cUPMDTk: "\/doi\/full\/10.1073\/pnas.27.4.222?__cf_chl_tk=JMpMDfHKl7cLAPjbM5gsahmSrEbojWdyddiFYkf.zuc-1742455542-1.0.1.1-ZzkHBl1q1VsJJ3PTtI8_kFbZLWtORIF5uj6_xWwB3U4",cFPWv: 'b',cITimeS: '1742455542',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/doi\/full\/10.1073\/pnas.27.4.222?__cf_chl_f_tk=JMpMDfHKl7cLAPjbM5gsahmSrEbojWdyddiFYkf.zuc-1742455542-1.0.1.1-ZzkHBl1q1VsJJ3PTtI8_kFbZLWtORIF5uj6_xWwB3U4",md: "sBguIuvg345j9.1oNAMiJJEfu.k4qacBop_I4jozW6o-1742455542-1.2.1.1-S6PMs6UvihV97AZiQYHg3QAUWjZsinjbbGfQg29QoA__wd.Cr9OcX4vmtsj0J0IkFV6Yu5v9RANnke2LNSRjX_5UekmgMB_q3woKkGCkDvEf7W7LOnnLuxfnRl3VP2KwKiXia1QVTOoDuw13ni9mz.lEm_uTHtEp.NncKV8VtHLLdEVJbNdsCOMqERP8p98FEKb_GgPvGSigVdo83_0ztslIAK4gJWwl4Jx.J9B81CJxK28lcUQ5W3CvwVxlRaAdFgCWPIlNlNmm7B9yxOmjffqwzWwO733ICixcYOviyu04bXYwUtE4_N5wjUqjhQ1SMUOENntxPYDAEdQ3pJEgnCQB3ARAiuXjI5cvjxM.WsWXHfN.DjEe2ohUTUbzRF35fsisHzllup0fRQbHinK0MXKEZTydNNrCu14y6ZVdgH8EgauqhjlCQMKEIuZAChbI9if6jg27m9tl3JDbV1v0er8QJo9wgl5MJQx7uMBjCmQSppg1CSuoq6_g.gQaPvGNxCbfOxht8OwK0MI..K1dG6TUALeA4tepFhp.lSJos2.LyBLWIvUHJfHL52ZEtanyzAse4TuuW.Lt5bbu_H29r4A9VpJgOClN4QLpKnVv2OKqpBx4GY5Rv4qG6SC00KjTqhJaAT_jx5YwFqA49xEwkPAVkns.0ly30zbsNZ.nj76g45APWFBIkXt1uHFz4_Npy4Sc9jbf5_kl6crL8HpzJEmFsAsyC4aUYMZCqgrfXpxBMFGnzmJ5gYTaoFhH1G3eTNbgrbG5pJYwIPwN77tnUPwxHdcJfOnaBHIsC9fADEgunC4kpS1xsRgC_swVkVGwelN5rPsh2wyQiwBlCaPrDi1j2mYiQ.G.ZQr8JqFJ43UW.ZPbKejBSLBgujBAzFx6dS.i59ZPP8ZwDHFH32vlTIlkS_Wb7qcqGMDyS5.5lFU2fGbH3MHKRTTqDkGWcqFbKkmzUPjzzHkIwXjdjkpshRcgtWe5ylsrNRjKcuo83YGU19Nr3VCew9qNzXQtAOJZ8KLyXyZ.JZGpIb3UV8cmWGi5qgCNzsil_tTE4X56OiYNKC17o7c5X3wWOYJWzA4ferBizG6AHrCR5Zsc6r38QJKyGMREa6Z6DQ4P2tULP.myxXtytKucdX5h28lM9NcPukYmLoQgaHb8HsuVOIQ6Oqe7wUGMRwFAyygU6tgGbxzoivDXGg8lX3FnyxaUtTgIZZrdYuLzjdYqwCgak0PMBzKESjz.kL_MJU3uFFx6qlMo1ZPcxxS7b0GSUWHGgtk8Gq_MihUPtaJch_vK90sVjg",mdrd: "JVvN_oOp50z9eIYRL4fWKw3gbVLKp3yPH2MWY6fyjkc-1742455542-1.2.1.1-OyTYxyWNKs3l3M_HOCuR.oxmPpm_ujO8U1FjVSM6hG4NAp5OK3qRthgWKKg.8spwkraZ_EOfz0xeKsNRwplCq3cqKi1NxaaKRoQ4GpQzBeTMenvhL9Z_kKIf2KJEbh.WmVErL1CKL0xMAW7uPQt74sloZ3CicRhWyr6Ty8FDD2vBuTiY77mepTPgtSM6rmMzlVj8cLte4DnxNJWs60RoH5N8aiJJ5QycqrewEQH3wBNc1zsEtoM4S9e1fJfpI8_oZz7jL6frmq70LxEDVuaf2ON8MW5UIHCp_opOgO43qQauJOSXsfFo8M1AbL_qH92S6jij4VR6yOGGTT3GbKsoIPTu59U8E6rz6NM0p7Ot6QrVerlnLkeDP0u8443hURYKj926NovPzZdG9qEM6f27nIV1uBzY7.QsShTSZCOykqqqOk_dNyTg2wEqGTxea9jEMkxItmyK6vGpxCMO_o1h7jDhkBHiqfB3ZJGuIvPOqYXPz4tALuaMuH2d8me2oaJhkLwlwT.mtCbV9ko76ni8PVQJdXk683MnHb1Rcy0L1OAqxWMBHou8av91Qojp4Abx50WKaTwdsb0Zoejo848yNMKD7mgaA3otVULaSoB8v4g9a9SuQPqgFXF06.G6abC553CpXdUKvB7ZRylHcAMXyqx9nmTYgN1LrCKS85y9wfGGMiy.K5AbQqWwSr4mujnuo0QBCAf58jxsQtdCNV0heT0qmSj8Ro811IuHub.p1BBiext3g00CcS64qenWVOKtWdZft8CUFP60f_Vcb5bA.TIqZEhIwA6HECNBGG7IkiZ0YcyfyHRoloOxDLTXgED9lqLmJ9wXvPXcKijMMkgR5tYcg5QCW1Ylo.PoHOGMEXfHyIyODzEFFfRkNJ.uuMb2Y.w2RJ2at6zNN1uk41kQgeUQDXbm_p5NZcQRHTektTymKpZl9OQbKTkBBi8xGvVtNeQ2gEnwrz.RjofGVejUFpGWp3DEHIQAWYldkLdibDaUZ9fj7eJc2yIOBcCClesfmYQ27xfFXLfz0Hk8fy1.pLVZld9SICRvs7m.SdsG1Oppf3Le5Kn336kD54yXGP0hnu10zc_TTU1WD2kssBhqlQS.SoaDT8iut1Kxu85cavpn6wWrKvR2YUAvHzJiaGh0cWVXHzRry5Lesr7oeD6W3BiZO88NpieOLpXXd7BMAWjSJF.jPzxUODEPKqouv.qqsXxcZSDKLtri0BEFzT7uu6U9JweWzpIJC1jA32XJySLKax.ZAPNnFhg2ZsMuPc42vkNI_UQv7wkqgAtbqgwiDfHSZg09fOw3OyHYJJWQVfNUrkWENZ1zhzNdl89DWv0Br5inB5rva8uwkpoZQfILlx3IoSN22.1punKsSnqQyS1hJNVHOfENT8j3DC_.f1S9ddv2hwjA1VE3.NV2ux8VgffMhAi4gnjS8rNTPaNJ68wex.dU7AQPsUc.d9j6JdD_kycvms0_uuqwHb4k_RYdDO5wF7qG8kBTEDBMF8Pvje0za5D3WaM6JbTpfeKqcdoiSJT1JBmmxlCTaXwvzYz1IE7druhJpIunV7Z6H8OnP.vcsNTkq7S39Qtj77GpNEgAHGIWTrquTBvVr80grai.oxG3uIxJiH8rLYnHynEFzwhBKTzwq6GKatqiyclxPonmST4IUbm0GK8k0_2JIMMIMGw878lHN9yiOjd914hDnirywy6WWJQkYP9DMMyYcC6lVEjcrSg9XeRZGFVGxIxkCU.BfCdv8Y9yxUhYkJO2QoSpqvpVfSZ9OiwSHJv87iExYew.H2V0NouIDOmV6HP4PMrnOaPpu7jA.J2gjS4SWyNXs5KjzFbnPpnfbsanvMm4pHJnTUI1a5tZKXNcCO7hHYe3nPqm6h.wdPyaVd.a5aDskPNUFAzz9PO6mr2gUjaKMH5GPM_3f8EdGKFxYbHurgVCpyDHehwEthvjpzonMB1nVe6Vs4ZpbC23eIYjUda75Ue02hE8.Hc5iXz_uyrL1TUVIuyAadswOY9igsJrVq3u_fn37B27DO8PwQeL9sApbjPymZrZDYPmqCNFURmIpg"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/b/orchestrate/chl_page/v1?ray=92337a251860919c';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\/full\/10.1073\/pnas.27.4.222?__cf_chl_rt_tk=JMpMDfHKl7cLAPjbM5gsahmSrEbojWdyddiFYkf.zuc-1742455542-1.0.1.1-ZzkHBl1q1VsJJ3PTtI8_kFbZLWtORIF5uj6_xWwB3U4" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>