Polyadenylation of the 3-prime ends of eukaryotic mRNAs is a key event that takes place in the nucleus during maturation of mRNA. The reaction includes endoribonucleolytic cleavage of the pre-RNA at the poly(A) site that leads to synthesis of the poly(A) tail at the 3-prime end of the upstream cleavage product. The adenosine addition reaction depends on poly(A) polymerase (PAP) activity. The testis express PAP-β (TPAP) in the cytoplasm of spermatogenic cells. The adenosine addition function of PAP-β plays a critical role in male germ cell production. PAP-β-deficient transgenic mice display impaired expression of haploid-specific genes that are necessary for spermatogenesis. GSG1 (Germ cell-specific gene 1 protein) is a 349 amino acid endoplasmic reticulum protein that causes the redistribution of PAP-β from the cytosol to the endoplasmic reticulum. There are eight isoforms of GSG1 that are produced as a result of alternative splicing events.