Many proteins involved in the regulation of cell growth and differentiation possess structural motifs that participate in specific molecular interactions. The human rfp (ret finger protein) has a tripartite motif, consisting of two novel zinc fingers (the RING linger and the B box) and a coiled-coil domain, and belongs to the B box zinc finger protein family. Rfp becomes oncogenic when its tripartite motif is recombined with the tyrosine kinase domain from the c-ret proto-oncogene. To further understand the function of rfp during normal development and cellular differentiation, we cloned the mouse rfp cDNA and analyzed its pattern of expression and subcellular distribution. We found that the mouse rfp cDNA shared a 98.4% homology with the human sequence. The gene mapped to human chromosome 6 and mouse chromosome 13 indicating that it was linked to a several other genes encoding proteins that possess common domains. rfp transcripts and protein were ubiquitous in day 10.5-13.5 mouse embryos, however, they were restricted in adult mice, with the highest level of expression in pachytene spermatocytes and round spermatids of differentiating sperm. The rfp protein was detected within cell nuclei as nuclear bodies similar to the PODs (PML oncogenic domains) observed with another B box family member, PML (promyelocytic leukemia protein). These results suggest that rfp may function in the regulation of cell growth and differentiation during mouse embryogenesis and sperm differentiation.