FOXP1 and Hand-foot syndrome: In light of the well-established p19ARF/p53 axis regulating cell cycle arrest[40,41,42], the decrease of p19ARF expression and S15 phosphorylation of p53 indicates that Foxp1 loss relieves cell cycle arrest in anagen HFs, at least partially accounting for the effect of Foxp1 in promoting HFSC proliferation.