Fig. 4. Cross between Msx2 and Fgf 5^go/Fgf5^go (Angora) mice. To address the genetic relationship between Msx2 and Fgf5 in hair cycle regulation, double mutant mice for Msx2 and Fgf5 were generated by crossing the two mutants. Hair loss in Msx2 knockout mice occurs invariably at P14 (A,B). By contrast, Fgf5^go/Fgf5^go mice grow long pelage hairs as a result of prolonged anagen (A). Mice doubly homozygous for both mutations exhibited long pelage hairs and no longer lose their pelage hairs at P14. Instead, hair loss in these mice eventually occurs between P18 and P30, depending on the genetic background of the mouse (B). (C) The hair cycle length in each genetic mutant. Anagen in double mutant is prolonged, similar to Fgf 5 mutants. Hair loss still occurs, but is also delayed accordingly. Catagen and telogen in double mutants are approximately similar to that of Msx2 mutants. The result suggests that hair loss in Msx2 knockout mutants is associated with a specific time-point in catagen and is delayed by mutation in Fgf5, which prolongs anagen length.