“…Although there is clear evidence that the burden of senescence increases with aging in human CD4 + lymphocytes, kidney epithelia, and skin, the quantification of senescence in vivo is complex because, in spite of the defined set of core features, heterogeneous forms of senescence develop according to different triggers and tissues (Koppelstaetter et al, ; Liu et al, ; Waaijer et al, ). Importantly, none of the characteristic biomarkers described above, including p53, p21, senescence‐associated β‐galactosidase, and SASP factors, are specific to senescence, and p16 Ink4a is not always present (Biran et al, ; Haferkamp et al, ; Laberge et al, ; Noren Hooten & Evans, ; Rodier & Campisi, ). Attempts to quantify senescent cell accumulation in humans from blood biomarkers assume that the SASP proteins dispersed in tissues spill over into circulation and may be detected there.…”