Plays an important role in the regulation of cell proliferation and cell differentiation. Required for normal regulation of the hair growth cycle. Functions as an inhibitor of hair elongation by promoting progression from anagen, the growth phase of the hair follicle, into catagen the apoptosis-induced regression phase (By similarity). {ECO:0000250|UniProtKB:Q20FD0, ECO:0000269|PubMed:8663044}. This is the function of ENSG00000138675 (FGF5, fibroblast growth factor 5).