Senescence is increased in Fanca^−/− MKs. (A) Frequency of SA-β-galactosidase-positive 2N, 4N, and ≥8N MKs per mouse evaluated in a minimum of 200 cells sorted at day 3 of differentiation in vitro (n = 3, *P < .05, Student t test). (B) Quantitative reverse transcriptase-polymerase chain reaction analysis of p21 and p16 expression in sorted 2N, 4N, and ≥8N Fanca^−/− MKs. The results are calculated relative to the WT and normalized against the level of Gapdh (n = 3 each from a pool of 2 mice).