To find additional members of the basic helix-loop-helix (bHLH) protein family in human, the bioinformatic tools provided by the National Center for Biotechnology Information was used for homology searches through databases. We report the identification and cloning of a new, fourth member in the achaete-scute complex family of genes, Hash4 or Ascl4, which maps to chromosome 12q24.1. The bHLH domain of the putative HASH4 protein shows most identity to HASH3/ASCL3. The expression of Hash4 and Hash3 was analyzed by reverse transcriptase polymerase chain reaction (PCR), in a panel of different human tissues and cells. Expression of Hash3 was detected in all samples tested, while the expression of Hash4 was restricted to skin. Quantification of the Hash4 expression by real-time PCR revealed a 7-fold higher expression in fetal skin compared to adult skin. Our results suggest HASH4 to be involved in skin development.