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However, there are few tools to analyze differences among quiescent (G0) cells, though such differences are relevant for development, tissue maintenance, and cancer cell behavior. Quiescence heterogeneity, also known as quiescence depth, typically correlates with time after cell cycle arrest, yet directly measuring cell age is not feasible for all cell types or most tissues. Here, we describe ELDR-Glo, a genetically-encoded fluorescent biosensor that estimates relative cell age, or time since the last cell cycle. The biosensor integrates replication-coupled degradation in S phase with a slow-maturing mCherry and a normalization module. We demonstrate that ELDR-Glo signal correlates with true cell age by both live-cell imaging and in fixed cells. ELDR-Glo distinguishes early and late G0 cells and functions as a relative quiescence depth reporter\n                  <jats:italic>in situ<\/jats:italic>\n                  . The biosensor is compatible with multiplexed immunofluorescence and flow cytometry. ELDR-Glo provides a unique and scalable tool to investigate cell proliferation control.\n                <\/jats:p>\n                <jats:sec>\n                  <jats:title>Teaser<\/jats:title>\n                  <jats:p>A fluorescent biosensor to distinguish cells of different ages and thus, likelihood of returning to proliferation from quiescence.<\/jats:p>\n                <\/jats:sec>","DOI":"10.64898\/2026.07.04.736063","type":"posted-content","created":{"date-parts":[[2026,7,6]],"date-time":"2026-07-06T13:40:11Z","timestamp":1783345211000},"source":"Crossref","is-referenced-by-count":0,"title":["ELDR-Glo, a biosensor for cell age that correlates with quiescence depth"],"prefix":"10.64898","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-0476-5567","authenticated-orcid":false,"given":"Martha S.","family":"Johnson","sequence":"first","affiliation":[{"name":"Department of Biochemistry and Biophysics"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0002-6868-8457","authenticated-orcid":false,"given":"Sapna","family":"Kamath","sequence":"additional","affiliation":[{"name":"Department of Biochemistry and Biophysics"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6475-8756","authenticated-orcid":false,"given":"Dalia","family":"Fleifel","sequence":"additional","affiliation":[{"name":"Department of Biochemistry and Biophysics"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Tessa","family":"Hill","sequence":"additional","affiliation":[{"name":"Department of Biochemistry and Biophysics"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0984-7166","authenticated-orcid":false,"given":"Liu","family":"Mei","sequence":"additional","affiliation":[{"name":"Department of Biochemistry and Biophysics"},{"name":"Department of Biochemistry and Molecular Biology, University of Nebraska Medical Center"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0008-1332-4246","authenticated-orcid":false,"given":"Neha","family":"Das","sequence":"additional","affiliation":[{"name":"Department of Biochemistry and Biophysics"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0001-3501-7553","authenticated-orcid":false,"given":"Mary","family":"Linares","sequence":"additional","affiliation":[{"name":"Department of Biology, University of North Carolina, Chapel Hill, NC 27599-7260"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Wenyih","family":"Aw","sequence":"additional","affiliation":[{"name":"Joint Department of Biomedical Engineering, University of North Carolina, Chapel Hill, NC 27599-7260"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2135-5153","authenticated-orcid":false,"given":"Victoria L.","family":"Bautch","sequence":"additional","affiliation":[{"name":"Department of Biology, University of North Carolina, Chapel Hill, NC 27599-7260"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0849-7405","authenticated-orcid":false,"given":"Jeanette Gowen","family":"Cook","sequence":"additional","affiliation":[{"name":"Department of Biochemistry and Biophysics"},{"name":"Department of Pharmacology, University of North Carolina, Chapel Hill, NC 27599-7260"},{"name":"Lineberger Comprehensive Cancer Center, University of North Carolina, Chapel Hill, NC 27599-7260"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"54368","reference":[{"key":"2026081715050655000_2026.07.04.736063v2.1","doi-asserted-by":"publisher","DOI":"10.1046\/j.1365-2184.2003.00266.x"},{"key":"2026081715050655000_2026.07.04.736063v2.2","doi-asserted-by":"publisher","DOI":"10.1111\/febs.13898"},{"key":"2026081715050655000_2026.07.04.736063v2.3","doi-asserted-by":"publisher","DOI":"10.1016\/j.cell.2011.02.013"},{"key":"2026081715050655000_2026.07.04.736063v2.4","doi-asserted-by":"publisher","DOI":"10.1038\/s41580-021-00404-3"},{"key":"2026081715050655000_2026.07.04.736063v2.5","doi-asserted-by":"crossref","unstructured":"R. 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