A novel mouse heart-specific gene, Srd5α2l2, was cloned from mouse embryo heart by EST database searching and RT-PCR. The cDNA of Srd5α2l2 was 2 348 bp and composed of 12 extrons within over 77 kb in mouse genome. This gene was mapped to mouse chromosome 5 by BLAST search to mouse genome. Analysis of the 3′ untranslation region of the cDNA showed the presence of ATTTA sequence, which was believed to act as degradation signal of mRNA. The longest ORF spanned all of the 12 extrons and encoded a putative protein of 361 amino acids. A conserved steroid 5 alpha-reductase domain (3-oxo-5-alpha-steroid 4-dehydrogenase，STEROID_DH) was present at the C-terminus of the deduced protein. Homologous alignment found high identity of nucleotide sequence, deduced amino acid sequence and genomic structure between Srd5α2l2 and human cDNA of DKFZp313D0829（AL833108）. It may be concluded that AL833108 was the human orthologue of Srd5α2l2. BLAST search showed that cDNA of Srd5α2l2 was supported by 41 ESTs from EST database, 25 out of the 41 ESTs were from mouse heart. RT-PCR performed on a panel of different mouse tissues demonstrated that Srd5α2l2 strongly expressed in mouse heart but not or much more weakly expressed in other tissues. These results suggested that Srd5α2l2 is a novel heart-specific member of steroid 5 alpha-reductase family. As far as it is known that no heart-specific member of this family was reported before.