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: "pnas.org",cType: 'managed',cRay: '912f109679416be1',cH: 'FqqdLCU72KOG3r.a6jSJkF8v2iUnF4Ao1tAF6AWjQM8-1739724921-1.2.1.1-y_bmpDuUM36a3PO9jf.GoseHyLa4R_PMeLfiRp94U1j3nw75zpBBd5rsdXvdtXWk',cUPMDTk: "\/doi\/full\/10.1073\/pnas.86.23.9355?__cf_chl_tk=vHufCWc2PN3wYmDjuZud_uNcCOrOeFU6mBbrpL7_tW8-1739724921-1.0.1.1-e2oNHMxc8GovugYtbH96Z6dIRZm5bTAh6wqwRc6p9F0",cFPWv: 'b',cITimeS: '1739724921',cTTimeMs: '1000',cMTimeMs: '390000',cTplC: 0,cTplV: 5,cTplB: 'cf',cK: "",fa: "\/doi\/full\/10.1073\/pnas.86.23.9355?__cf_chl_f_tk=vHufCWc2PN3wYmDjuZud_uNcCOrOeFU6mBbrpL7_tW8-1739724921-1.0.1.1-e2oNHMxc8GovugYtbH96Z6dIRZm5bTAh6wqwRc6p9F0",md: "A.79grxXb7bhqMbbRKfAT_xFDTDvdhDLwCKGtUVKkNE-1739724921-1.2.1.1-_4dW7lbpQHVCYmRDNj3nXYa9IA_FHjALws.NGp1J4qHyP_zfsQTrrsPlSmnyW8k1F9ltLLTjZBDW339QK1kdy_HOdKYC05lRuJDGzkuaAfyNcslU43Ja3LS5_uRQaTKdDmHDffHqlXl4mtTk7jhuUuv.FZZE02_SYAKTImhtHVaZEl_QJBR43vQkGVe0Wtl4im6af3lXY9GRr1ueez_0i6AeFyjnG9jiDenZ65fUfpoiH2Q4NfUxuXG5ezIjH1PCodfCx_V16jvD9B8xiODRvg4gDSUxLTpG5OLhdJjnsppi4ciki2m0GPW_kVjIbevDdENJkZHk56mOSaWLOoR378b_QxYb9pXFw.XYSj1DNm5vX8YjkepCcxa7GkK_0SPeokH1sEepYjP9CixCtskf4QjqYJ.BM_vN5RCP1i5hfSbl_vYf9aaCzzJvxx5llUxUTxiV7OjTR74UU3n_T3MvdMNKOZi6ekUu0B05IHuWb6Sm7U9SLV.ptU1OQbLuBpbslEWYtpB7f5yIjs0iN9xx1AtI2Q5zvmPswc.tLI3R8i_W_Id92f6TlVC7eK4XmfU.usLmBzLUupfMkC5VahhJFBA5V5dPzRct9SOAEeXfAJWttErKl1XnPTl2XTp4c1DPIo23rmQM_lzkhnrFB56Hjt3L0xax2kIcGUQmL.G.xnlKP7c6ZwgBS6maQn.d_fFuZH2AuDGTt2IXMf0xEhJsphHtlBpbwlcpiKnde3zBBTUZlndxNGGbc6JfyoNPfjxGvpwvu8kPrqPNiNzljpT8fEymn8YuZWMeLFJv1PjWIMeOifpFYnu6wbsLBQVdRc5zc04yV2iG7b1d2S0SePVT4YA8R6MrJCCjFUbKL8DEqtqJzlteFYTevFk5J0U1ZOd.cFMCC43H2yctxw729H8ANu2R.zi5uer4_hq6yXQB0p0MIebwTdWS0NernvjniI6HkwOVFW1xkU9pFanLtT61AycZM0SKl3nyrCMiFV4nqcx7tXyGp5oSzF7z4PPXDY8zhmHbqA6E14JGQHvCetRr5Yg8QDHBDJDyBEpdTBUJzdH_4u5FUINR0VC6EVwh6xn2pSXpooS__3GrwCLiCLbgUF1pPI4ZXvrvsO_3ZHXPl43G5qX36TRxavGDqh16ChgPdnAiek4WvgeK0tgDeL8F7APGyZe5XDsdp8OqWQ15NvVvYVXw_TIPfQ0XJVND6DlxwFbyRH3RiSSELmxL_YUbJZgjN9DRMVniZuzNNKgb4lbGX1IfKp0KmpFktBbY29y_za8vlpGk_181nuoblpkd2sbFdPyc59hVhFrEL3lV8djEeF.TsRjlbvY1tulJ89_LR_Ad78AWrFF_AAIOwZSR0HRcPq30H.h_.zFfjNGOsHu7gqKgTN9_ASjxDYflOt8KUTdXHJlvnAYHVXt52I09hCNGnvKy2O7IBdLsceY0VFTt32icZkJwLDIQfR77ts7_bkdHQ2Ua8kLopVRdXJ0w2rX32CfmzyFaQwFpK0Z3q43jPptwxukAzqVY6hPW2pAn6Vxe9E1lcqxNlpTjD_ucpaFRvouzLzXHDokEAK0CWLoPlYhrHzYp2h8Nbj9knM8Y0DxTBhM1GCUVbcNm7SJifIkqisRjUE6E9p5l8hsucRmrvz_tgeNxeQ9CUw8Ax2hLZHeg1toRMD5JRmeDlozZOWnzL78d9QT5UJVRLjq4wfjaHd3tXfuthJxgf8ImLBaOt9dgobcG5nonpSKN06Be55sED2lUSMP9l7iJBqEq7lwm3QpahiStcryNOV49NVHyTxbF3087Hd.oOds8NpwOG.SEXE4AThDEOVp.z8LfBmZBxMIG1Pljk7kbJDqMpasPT67I4KtHuItb85QM5TYm7eqSG5AMQNKEeCUNLTYgcHlujJSPPlTmQyVCL5eCAJEImidrLWuzWsGLCC3M03B75e8VKkRQ9RxGT1IgbUxVfBBam32LiHbF4xH1cUCtI_a9X2s2iwz_2pwDECScb0KAM7RE5re0xeuIfJdOrY6qgFAvz09Oc84zUP04YJo6nlv8OoIQ_.dRs2VN21wrQaNi7_BhgeVgx_..FbocjGfnlRrTWyXsdGQRY_D5RbcAt2dXxMTH5toFoqkiTEmgukTXBdsTQspAaPelbVNpeaWONgeNYDJ6rImgJg68uU_1a9HKd09OAxCZlQv8fG9UDs0Go8C9VXOlYdh2mUFSeJAq1Ts8Ksol8jH1nY6Y8nU_ChiJoe1vIs2CLnuZgQx5YYfCIV_GwAEXQI2M.XI8iE0JAXMeMrqxl9JLGd2BdPLBl6Hy4Lh5eS8yVX0bTR1RzsJf7LEBzy_LjdOqwlHWSwP0rHKG0tFx1tSg21MjKVXZX.psZIjTEe38o.Loz0i9Q1RE5e5k1zS89T04xrBsmikw.biuqJat6nZUnXa.ZOdNJQHDTaYwoPF0uR5HdVSJFwS.bbgU17aZY7EpdQYT8U48_pOsWKjYZsLSzZWFRwc8cbLv3VosuXep3628CODVBfR1k7V2JHlW.S0C79q8XdktcJ9toinE36F8uI.kXHez22dP0AhlWMtdw70iPqD.G.2X.9E2SWPwqAQ7fNv_Gh97g0o",mdrd: "TkZ9GAd81L0RY3LanrhmWzvITjvqel7tCY5ddI8tKWQ-1739724921-1.2.1.1-KtrjI9xeKOg2e62Yf_1hFPp7kNMx.LEuHb.qwW1tPtkGCAcZ_74SyY3e43.EMMuJciH0Q9Cadl8qTk49npABbJfYPybSxGs7ehgwTBB_8S9KINA9_qDU1oMdsnPnXxyU0TZuzRA9XMToIQLj7ZFO_c3ixzoDoKPuhTIFX5cKo71f0Q06grfSBYgwmx18wgRaNBgsuBrsli5jNFSzJfY3MA_G5T6k3YGeLLRPCQ_IFFpeBs0vBXmYSpFmpCDe6hrQa1ekui5g7RA8W8sVxOv.T0Jw89J5XfC7XTbhQrU2OnggVlH5sG58T8O8N0V2hidV2GpxWkimpF6lX6UZP25Q9U2O29Ck5PR5XtuY9NRtPsnJ_C2EoUP_t3QQaKXG_rBoGaMgnzp22SFW7aZudULVMaF8QveRq8mZ7bSOdyaKBdRelusE8aQ1dEt7eTn433l.JDTPqWND5RMyn.IYTiPw31.LDtA3J6R5hdo4WC1z3kOrD40.2_4emh3dbo8x0ncuoQO377KsUzEVjy2.wrVSExm_lXPB0058RtBealVny9OOscn7sAKen_v_NQz7cbSWeMi2vAD38WkgSf_FQ9x9.rcHTy6ADpilv3Kxse0SdMVhp8akfMqcVWxoC3AdyhPK37rQG86L0upG3NigTcXmsmkHKYAUZSpVwLCOiEcpBSVSZ8u_hIKi2u8orJnLImBBNiWwRECW5uCfhRmaUAtFQV7W3EbKCAvw5MF57yhjY9VjRjqLXwo9JNKwltEq3FzZ3Cwu2R94l3IwgyJlM41dU_sT79ewkeowfbm2mmOFmnUzs_pJ9di5.qJn9X0LlpkGOkM_Uz_YHianfLF3rMYxkBLujYXSsxmzyOQuexwNsxVWrss0IfSv5rfqJ8qDanCNNPyJcYNpuqyD2nTImsRCD4QG20TNN2ZeVCVVjSGfvlPttp9rxj0qgoRDcr_oz_uUYvInZVxE.SiM7WOhzN_I7t1N1vHeiHhBKfpw464mQJDR63EfZM2_C3NkWX4CeWUCsVflIiJ6gUIaeIB8UAJIgLiVjxIJuptpN.gNlJInTu3qEd8vxoW4LcL05Bv75yIP54hSLMh2q_bnXGqSfN8roCKBTEvbt.mAAI3P1cIYBzbDREgXrhwDreIeOKqNwgf0b3jvVFAojFVGWAEL9rJkqrMeEnGU6yMslLQ8bDN0uJjdCRyO7_AOod0Pe0vJR8VImZsBes1k.VOZTahD5fVAeLWdvSccoLfMmZrRMyNMy3N0hTydunZfpwdsHQeJ5olSV9B5lgpadPP3OYEBLcVLYbqxlmgugBV.feVnD8imnvhn3C3F9iN9BRM3wU4_nXv2e_ZHuUxp0qwlxd4Z44S3J8JnF2tcYdSw1CgfLLqR67CcdUy3qa3p7RBjgyRqf9oz2VuUzrY5iH1tOp7Ikn.1RWEIjwI6PG7pymVoBYghboGddfb4IRkBpplVafDImvaZeNxR1JZNEQ5rhrabdutdW8RZyRU2iNE7OKVTqj7jARiB4LaXjmrbe0za6A5I6wdfnVsMkenCUeS1ld0FIpwsGzmcB4y5Li6zUSO4kt5ohIOOgUqc9irIyJOEwERWKgQi5jPG73Y1u9_CCI4AoLB231i2QIx1QHurX3n.3yqLtfD5mlOz.dSzNSb4XYJi8Ym50LSgUEqMCrOFn6yWPJ6IBg2xQMbb72ptiYipbXC._pUODhKymMBYUHkcuWBJ7Q9klHzeUygyREJKjChz5owQ0UqNV2JiYQJ8qAaHCMKfUs0OpeJd7u2u7JdBuJ.6fF9njas1vuiyiJpsEbFRLPi2twYgUpT3Xr24h9PVTUkolP12eaUxfPtCQn4yUMkD9XLR_AItE0PqyQQwVz3U8KPIaQKUZJzkIf3lt9O.pCOx9oTBOylb9VFqM3H7_XeHLXhN_tzWGaYJvW.ZbsvI4Wau5LDa4bEIJLlu8NwrMdRT4Ck"};var cpo = document.createElement('script');cpo.src = '/cdn-cgi/challenge-platform/h/b/orchestrate/chl_page/v1?ray=912f109679416be1';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.86.23.9355?__cf_chl_rt_tk=vHufCWc2PN3wYmDjuZud_uNcCOrOeFU6mBbrpL7_tW8-1739724921-1.0.1.1-e2oNHMxc8GovugYtbH96Z6dIRZm5bTAh6wqwRc6p9F0" + window._cf_chl_opt.cOgUHash);cpo.onload = function() {history.replaceState(null, null, ogU);}}document.getElementsByTagName('head')[0].appendChild(cpo);}());</script></body></html>