In familial trichomegaly, caused by loss-of-function mutations in FGF5, body hair and eyelashes become excessively long owing to a prolonged anagen phase.54 Similarly, a prolonged anagen phase is also observed in Fgf5-mutant domesticated animal breeds, such as particular breeds of cats82 and dogs83 (Figure 4i–k). This evidence concerns the gene FGF5 and trichomegaly.