Many mRNAs in male germ cells lack the canonical AAUAAA but are normally polyadenylated (Wallace, A. M., Dass, B., Ravnik, S. E., Tonk, V., Jenkins, N. A., Gilbert, D. J., Copeland, N. G., and MacDonald, C. C. (1999) Proc. Natl. Acad Sci. U. S. A. 96, 6763-6768). Previously, we demonstrated the presence of two distinct forms of the M(r) 64,000 protein of the cleavage stimulation factor (CstF-64) in mouse male germ cells and in brain, a somatic M(r) 64,000 form and a variant M(r) 70,000 form. The variant form was specific to meiotic and postmeiotic germ cells. We localized the gene for the somatic CstF-64 to the X chromosome, which would be inactivated during male meiosis. This suggested that the variant CstF-64 was an autosomal homolog activated during that time. We have named the variant form "tau CstF-64," and we describe here the cloning and characterization of the mouse tauCstF-64 cDNA, which maps to chromosome 19. The mouse tauCstF-64 protein fits the criteria of the variant CstF-64, including antibody reactivity, size, germ cell expression, and a common proteolytic digest pattern with tauCstF-64 from testis. Features of mtauCstF-64 that might allow it to promote the germ cell pattern of polyadenylation include a Pro --> Ser substitution in the RNA-binding domain and significant changes in the region that interacts with CstF-77.