Fsrg1 (female sterile homeotic-related gene 1) is the mouse homolog of the human RING3 protein, which has been shown to associate with the E2 promoter binding factor (E2F) transcription factor and to have a possible role in cell cycle-linked transcriptional regulation. The Fsrg1 protein is 60% identical in sequence to the RNA polymerase II mediator subunit Fsrg4, another member of this subfamily of double bromodomain-containing proteins that are homologs of Drosophila female sterile homeotic. Antibodies against murine Fsrg1 were generated and used in immunoblot and immunoprecipitation experiments to identify proteins interacting with Fsrg1 and RING3. In the presence of acetylated but not nonacetylated histone H3 and H4 peptides, RING3 was shown to interact with E2F, mediator components cyclin-dependent kinase 8 and thyroid receptor-associated protein 220, and the RNA polymerase II large subunit. Fsrg1 mRNA had been previously shown to be expressed at high levels in the epithelium of the adult mouse mammary gland. To determine the physiological relevance of these potential associations, we examined the patterns of expression of Fsrg1 mRNA and protein in the adult mammary epithelia during the reproductive cycle as the tissue is responding to estrogen, progesterone, and prolactin. Changes in the nuclear vs. cytoplasmic localization of Fsrg1 were observed and correlated with physiological changes in mammary gland function. The observations suggested that Fsrg1 may be involved in the transcriptional activities of genes involved in proliferation of the mammary epithelia during pregnancy and in orchestrating postlactation involution and apoptosis. Localization of Fsrg1 on euchromatin, the transcribed portion of the chromosomes, is consistent with its hypothesized function as a transcription regulator.