We previously identified a novel pregnancy-induced growth inhibitory gene, OKL38. To develop a rat model for further characterization of OKL38's role in the initiation and progression of breast and ovarian cancer, we now report the cloning and characterization of three novel rat OKL38 cDNAs that are derived through alternative splicing and differential promoter usage. These three transcripts differ in their 5' untranslated regions but share a common open reading frame that encoded for a 52 kDa protein. OKL38 is mapped to chromosome 19 spanning a region of approximately 15 kb and contains 8 exons. Differential expression of these three rat OKL38 transcripts was observed in liver, kidney, ovary, mammary gland and uterus. In situ hybridization localized the rat OKL38 transcripts to the luminal epithelial cells of the rat mammary gland and to the granulosa cells in the rat ovary. In vivo studies showed that the RtOKL38-2.0 transcript and protein were regulated by human chorionic gonadotropin (hCG) in the rat mammary gland and ovary. Importantly, over-expression of RtOKL38-eGFP fusion protein in Buffalo Rat Liver cells resulted in growth inhibition and cell death. Our present findings suggest that OKL38 may function as an effector for hCG-protection against mammary carcinogenesis and the availability of the three rat OKL38 cDNAs may help to elucidate its possible role in cellular growth, differentiation and carcinogenesis.