MAD (mothers against decapentaplegic)-related proteins (MADRs) are intracellular components that play critical roles in signal-transduction pathways involving the transforming growth factor beta (TGFbeta) superfamily. Some Mad genes are candidates for tumor-suppressor functions. From a human fetal brain cDNA library we have isolated a novel Mad-related gene. Two alternatively transcribed mRNAs encode deduced 430- and 467-amino-acid peptides that showed high levels of similarity to MADR1/Smad1/hMAD1 (about 80% identity at the amino acid level). This gene, which we designated MADH6, resides on 13q12-q14 between BRCA2 and RB, a region that frequently displays loss of heterozygosity in breast, liver, and prostate cancers.