ZFY, a gene on the Y chromosome encoding a zinc finger protein, has been proposed as a candidate for the human testis determining gene. Sequences related to ZFY, called ZFX, are present on the X chromosome of a wide range of placental mammals. Unlike most mammals the mouse has four genes homologous to ZFY; two on the Y chromosome, Zfy-1 and Zfy-2, an X-linked gene, Zfx, and an autosomal gene, Zfa. We show here that Zfa has arisen recently by retroposition of one of at least three alternatively spliced mRNAs transcribed from the Zfx gene. Zfa is an unusual retroposon in that it has retained an open reading frame and is expressed, although its function may be limited or altered by the presence of a potentially inactivating mutation in the third of its zinc fingers. This mutation must have occurred at the same time or soon after the retroposition event as it is also present in the Zfa gene of Mus spretus. Interestingly the third finger of the M. musculus musculus Zfy-2 gene has also sustained a mutation suggesting that this gene family may be rapidly evolving in mice.