The angora (go) mouse is an autosomal recessive mutation due to a deletion of ∼2 kb in the Fgf5 gene. Phenotypically, homozygous angora (go/go) mice have excessively long truncal hair and they can be differentiated from heterozygous (go/+) and wild-type (+/+) littermates by 21 days of age. The abnormal hair length is due to a three day prolongation of the anagen phase of the hair cycle. In addition, widely scattered hair follicles produce structurally defective hair shafts that twist within the follicle, causing hyperplasia of the outer root sheath and epidermis adjacent to the follicle. These follicular abnormalities were exacerbated by immunohistochemical staining for mouse keratin 6 and were identified mice at birth (genotyped by PCR techniques) through 30 days of age and in aged mice. Moreover, the long truncal hair phenotype was maintained in skin grafted onto C.B17/Icr scid/scid mice that have normal pelage hairs. This indicates that circulating or diffusible humoral factors regulating the mouse hair cycle are not involved in this mutation. Abnormalities in angora mice may help elucidate the role of FGF5 in hair and hair follicle development as well as in hair cycle regulation. The angora mutation provides another useful mouse model for studying hair cycle and its modulation.