CINXE.COM
Just a moment...
<!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.pnas.org",cType: 'managed',cRay: '913874ae0a61fd37',cH: 'XOlE29DVe2EXS4QS0U4STltrTkrqm3NSlL_AaXslZCI-1739823392-1.2.1.1-RQKbUJJRKZh6Nf4vndKelBh.2.Wk3xJZJWh5ABv5s5TZ_n7rlePcjuKY1Sp9rF9F',cUPMDTk: "\/content\/104\/36\/14277.full?__cf_chl_tk=OSb_bgIS7lQhmjob6iSdts6xsrAyZPTazJFy3p_EHDo-1739823392-1.0.1.1-sxBcFMspJX0G1zwXqi0nEpZ_CbVtPUS0bVTmBXg5Y8A",cFPWv: 'g',cITimeS: '1739823392',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/content\/104\/36\/14277.full?__cf_chl_f_tk=OSb_bgIS7lQhmjob6iSdts6xsrAyZPTazJFy3p_EHDo-1739823392-1.0.1.1-sxBcFMspJX0G1zwXqi0nEpZ_CbVtPUS0bVTmBXg5Y8A",md: "VQowUUeNUxhMdqoY09XdNvZBVZfaAnIi8xyIXQuzg_8-1739823392-1.2.1.1-cFrvXOJNmBiKSxRGVa1RR1Ow8xpGPeKmiX2Knv72Tml.bMnTTl0L_0UownZTuntJszu2aHAk0ldkSyO_6yksNES7awtr8ZZuPlClt.c7gx1WeSJFstluB.dP2.pZaVRhfbUdGCFhMKiQfTrl98POROHmhKPb4qbb2KizrxPAO9IBr4BjNGtvN7W0OHGCcDyG87lGaazdWXfVf3yPrc8SR5dA6Ma2Z4I4goENMwTfYsJy1OvNpOJi6TTfaDddAc2ZYW10u0Z7qFkGnw7d41IT60EZBOYby7D.h.6bbXfp279_.R.eViWvwbH9ibznxFI2MwM1wcuNkSS43GcpG4S8zbiU_xBy7G5QY5AhgKfSWJELfTnE7zhZOmBIQp23Z9BxRnA4vqoJKTKsTztuPv5VWvS19Z.1PnP_NM_giw7iDu6DPysZEbE1R.i4w2uoZYReggv.2ubh2oOsmG9_TPCVpJzJ5w33DaEA3VdtM.03_3d3PCSxwWMlZ5ZNh166gozNzjAzyiA45Zqt5peWGqRy7Ok8T9eOmMi_jlg2TSGSKVnKYcJV76U6BGKdXlljppBrq9kzlahnjQ2eVY_eLOI3dLvu7RP7AzFC8s7VP.cXMA4kj6lqdWLsEyELi2NhnVGu1XOBXmV4eDtN3WSqWQzmn2TrGigVpRqQ5RSfVLpJHqDJ.mpP3le40QWzPqESLoOFUgYKkbUDNrXXYRuswZnbgeNUO8_K2WNrHF3MW.P_Jl0vgzNonzVuf4BLh1IpFW0RhGcPB7kQaxTwV0oIsxRehwn1qfpmZT5uPJOz1v6q7kXwDStuStURz47DvLZ5sG.Wg.INKWX_7fEoJEXK21TMtfdCVPq0465WZN6zhgi9r8jzc.iEj649H5S0bGYv8LzSH1qAsRFmj8A4RX.HiuureFnwuQlw4JTM2Z_wcpNePcrgiPprZpzKnFpF18XcnpW6ETf5NpLwrPSrD_1_r8YIt.cOzgn4YXdZrmkEqAtUz98W9iS5t1Tnq4VTd4efKxJejmwNHC5qraNV.KffVlVRMq6Yue4wo.zNl76QaCIkBajYtntLXmanCAkDkbEL5CFSQaeyv6tR6_6qmaOn.Yd6hzp04bop7HTk1PbTPPcUCWM.4hKrOx2YMG4Xim39MjrtOvTMmwGjD0n8_2NvoWWBnPmU6RWKvL_6UuujPqvdMNavxir.i0L.49KDB7YlzwUHoQCkOGINH7O5DPPqp32jyn2UmcfC8RdsdEs2ZB5abArya7thDEjgf3ihuZig.KICby4ERbvFnyLmAFG8CxkQasoTKQxCT96WRVfpO0IHswEj_yVZutcaNGiYxAFKEZWV3DK0NIQugFH3dZfVgg8MqANztk1wS9Bs4lG8EBWfdW3dldiTs2PAXJhvW6wu_4EjZMbIyZb328rtoyONzZzX1AknGBUqLsH6CsdO8.xqjroRpTjOaUCWY97CvqeI5W.jOi5U4kKUJMN6kLWd06TTFpruyJxxx5.BDaZHxzmlKK3V40qKlKUr64hm3j1Md5tkwVz.mvsqrh3q318AHHuV.227VzYwSH7L8vp5s4Aq2LgNgcMjwJ0V0CSXGgfY4cpQwyxTC7KcgtVM0ovWyIHCwag6VtTWSqYmwEPdepwQBfvzdYpjKdJF8ytzefn55FtmFjF1z89ygYHhAoD.KvPfR4MH4N.FedoHZX0NaD82hDeknN2yYizJSTbvEAiIfZ8ix16e97CO4ICGHfmjq4sXNCunGf4ftUadeni8CcMx7qdI7WffbZLc.xMavCI.80r5Y.yZohD0hwPKXtKpYSokJ_has7bkIYyV5Vdtmn4E2ffHnZJioip.I8LShTyrxX5hUZUZpqyUJY6FxAZIBLQYV8LTdlf3PHXvP4_TvkNLZ_c.objEjL43pDECdEMp9NwheEE4E3F63z6O1Wur7B0RUePodqnpbXLrhErRGgSWNCoS2Tl59BJbbs2q4xAXpA_t65iLe479PwLiHJEQ2IwyieAI3EcoLj_vBa.GBTwH6aEUt1rl3sc5ZxbaBtgwbbYQil5OiUPq.gOflgdEHHMfZM0sQqLSwj.tlUuk53iMZckPlONUuuHxNj1wWXI93UScf6hwcmgEy0E9oiq4jPQuj7_OLQwyfdRtn48e6RZppz7SX2fy_z1AelSzeJ9.n8.o5h9sE_RI2wAKtgIlqE.X4DnNdZVcj69_SqQa_5CR9mO3nk0VE4FqcK_RHPqRain8yW9.zF_gPl95pSSVbw16DyHzc_ESIhRKyALGK0._uk4CmNklt_ohvVSQW3rZIxQwFAU7aekPHxaPVdDbO1QSpNk7hX0.XyuexgyWHupJjQOZbtMDdW9ZpIdehUExdZ7ZgLtkpqMNpVViAr.zDfGaY49TEzyPKCqhPEuAZ_u5sZW8iopOcDZ9hItxf_PUcsB_oK_sDKdWPy1wLw1fRwkZffyFcZlXK6v5yoF4Mzkrk7WQpH7giz2CyX2jEC0nHrCZfJHb720YQvqOwQTXa7n391e8moSga3g6F1ndLR0d0j_lkHi3iSZHpuSkN322Hz52pFOwx54nW.QFWxzKeN_VRA",mdrd: "bbg0ozK5J_edzrvajDlqUPlURvqropighv298tgqbdY-1739823392-1.2.1.1-BmpRgB83j8gmanUx._kiGIn14Uvv7MyaGoXWVVINrlr.pSjvG8c3scD.Khgszn4hLPQE.A7MxPgOL4aY301sHM0dClGLVwct2eWq_V3Md3ZY5BjYmslJH45mK.gUyu.M5y9N_7qK3oBT7ZOeaZWQN_t8dxT.7Xa2J1Xwb9BdBcuku6mKzi1ZWWFakgozWkb75o3O_27nksedIRCu2BgCmO70LR0feWlZ5QmYYReSNm7BjGmsRL7U0UI4XwSf4rJk00YPiA0TNlhEoN0.rXrglIqfFazlmYHbN1Y_4abb72VB2HpdvJ3R.46dgJG012Ij8OaICeHEH0dcy_1GufcrcA17IkU_g8TCkeubTE0TuoENRFuef2g53dVD7avPdbEprUyq5d5sFOetfAQvOMIYV5SYhSaCRyT3VOVnPLw69q1IWCjbYkHHCtvY6bC4o0WoSVD8mQ90.PLTTHDwwUGNG_dBwhcuU_gYtkF.4jm2.09IULyd9ja4BFj8JpjK4D6Ts.nHDH_rGAjX6TmYMnesNViqQybJIahppb9rSUw2tOyaYN3SP5gZv1yvBfgv6ng1QMLuEfDl4WvFG.LtT.KTnCc6TLQdhTDkXqWpZTm3uW0vOU1lE9gZQpLP6Tg_0fh0VpUqOA2D30olAW58AnhxMKTwQI_6SdOp4KcWTCYBHbQf9Lk0wNIcfhTL7dQ9KFHd..dGqujs0XG1D0JQvUE4WeCGkcgRB4cvFIX9HU9V8pfIfZ2HJzL5Zg2igzsBOvNJMZNJb0ll3ZtXC5OUws5hir9bHBiQPQADRsfxcHcvSSzltOC_1654g7ej4AV2_Jr7Sx2Q9nyceRvvEVvZ0fWRifbScSVJIV0tGOVHxyyj6RCH7TTNI57UaVuczWKoOdXRWtV79pwcBRwcAf.fNVYmDCWvcz96EpESmv900nhWNx7CI0zXEQSFGwEuF7jwmdWto3LFpLyaIX6sH6S26lCM41inWPnRo4Ep97p_zZ9GM4i.upKMoBwJObvunnfLXr1pDXkkT4PQ0lmsG9wgLmHurJqC1AAjamZTiV9Jo2sZic9EapuGpl.UbdaMzhpGbGy7TaFnSlLd9aoouYlFWgkDNi71pvSyisMvYnUUDjx42gH_2Eu2Wig8B8f41GUWOS6wYyFyJrvYmdL.XRo9Xmv5jYhqYoaHS2PwFChJjnrmyN1XS0hzAP6jVlddG4SMKIVVUPAeiWJxLLWPiyZPinvvGfzsGA5sEqudlWgUz6SxfBHpghCEJsGwoVfBZZ9UwLdH1MUVQnHx.uQZKr_PVPXLRs_jen5RtIFw0Ic_PlrXya3SEFUeCrb1PIT00vJAF6BCHI2TxGKUnT_pyF2KpB7MdVsrcCGINldTAmq_sDILIGiwWzKZtalzKQyqloq63VUdtFHwHORXMEdWeRQzWZjZHnPFwrC_b96_vIJDhf84KGiiZoFFr0.Afxno2_WaCNlKbUak08yuDhaefZ9OFKS_lI5jF9.TdpvAejVfvs0ST7cGQXu5R_knj.nZiRtlZGKhAA9rcldJNCB.QjjBWFc_8djJvAiv.QzzrmCqLnOFlyYZIya0Xl8CRuoPMwCmz_hd0O1.qYmXVQseibp1T2mNkhXbJssyBVXCduJT2r93ASw_m4OKQIzNEJ95kFFNFTuRI7KysUd.7oQneMzHyjK29qJZrYW3Hy.1SWvQslINTUKHXQv4XAJQS4mN.LkG5iVh17b7reUEppSJYfSXAdfPb2DVzuPXsbsn.yx1x35ZqrIKOXW2SWM4xhIjVX7dRMvyXNzAi6ZzdRp9JjeleP_R0YDw5eWiyoUuPwxDgW0H93nIlKGe4YuZA91KIt9N9vCS2E1q2FXBntv0k5BWAP.uMUwJGOwcsWYNlBEGwl85P0N5GtbqDCQ55NyhUCQB5OsfKSMsKa5XVfx8e.k00DfMBgOoeEsh7FOHKWQlPDONSwc"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/g/orchestrate/chl_page/v1?ray=913874ae0a61fd37';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, "\/content\/104\/36\/14277.full?__cf_chl_rt_tk=OSb_bgIS7lQhmjob6iSdts6xsrAyZPTazJFy3p_EHDo-1739823392-1.0.1.1-sxBcFMspJX0G1zwXqi0nEpZ_CbVtPUS0bVTmBXg5Y8A" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>