TGF beta, initially described as a factor that stimulates rodent fibroblast cell lines to proliferate in soft agar, has been shown to be active in several biological processes. The in vitro biological activities of the closely related molecules, TGF beta 1, TGF beta 2, and TGF beta 3, are comparable. Northern blot analyses of adult and embryonic tissues have shown the TGF beta mRNAs to be expressed in vivo, yet their patterns of expression appear somewhat different. In addition, even when all the TGF beta s are expressed in a tissue at the same time, the expression observed has been shown to be localized to different cells within the organ in some cases. This suggests that perhaps these molecules may have activities or functions in mice that are not apparent in vitro. Several members of the TGF beta family of genes have been mapped to mouse chromosome locations near loci previously assigned morphogenetic mutant loci. Although the relationship between the TGF beta genes and these loci have not been proven to be allelic, they may reveal important clues to the true activities of these molecules in vivo.