A new isoform for beta-galactosidase with optimum activity at pH 6.0 (Senescence Associated beta-gal or SA-beta-gal) which is specifically expressed in senescence (the irreversible growth arrest of cells). Specific quantitative assays were even developed for its detection. However, it is now known that this is due to an overexpression and accumulation of the lysosomal endogenous beta-galactosidase, and its expression is not required for senescence. Nevertheless, it remains the most widely used biomarker for senescent and aging cells, because it is reliable and easy to detect.