AbstractGerm cell nuclear factor (GCNF) is a member of the nuclear receptor superfamily, which is expressed in the adult predominantly in the male and female germ cells. In the male, GCNF is expressed in spermatogenic cells. GCNF binds as a homodimer to direct repeat response elements of the consensus half‐site sequence, AGGTCA, with 0 bp spacing (DR0). Using this information, a search of genomic databases was performed to identify candidate GCNF responsive, spermatogenic‐specific, genes that contain DR0 sequences. The mouse protamine genes are the strongest candidates identified to date, as they are post‐meiotically expressed in round spermatids and contain DR0 elements in their proximal promoters. Previous work has shown that both recombinant and endogenous GCNF bind to DR0 elements in the mouse protamine 1 and 2 (Prm 1 and Prm 2) promoters with high affinity and specificity. The present work shows that in transient transfection assays in GC‐1 and JEG‐3 cells, co‐transfection of a GCNF‐VP16 expression plasmid with reporter plasmids containing either the wild type Prm 1 or Prm 2 promoter established that GCNF‐VP16 is able to regulate transcription from both promoters in a DR0‐dependent manner. Wild type GCNF, in contrast, acts as a repressor of basal transcription on both the Prm 1 and Prm 2 promoters in a DR0‐dependent manner. Furthermore, CREMτ activation of these promoters is also repressed by wild‐type GCNF, indicating that GCNF also acts as a repressor of activated transcription. GCNF therefore defines a novel nuclear receptor‐signaling pathway that may regulate a subset of genes involved in the terminal differentiation process of spermatogenesis, exemplified by the protamines. Mol. Reprod. Dev. 68: 394–407, 2004. © 2004 Wiley‐Liss, Inc.