In an attempt to investigate the genes expressed in the anagen hair follicle, we differentially screened a mouse anagen skin cDNA library, and identified a cDNA encoding a novel protein containing one Src homology 3 domain at the carboxyl terminus. The predicted amino acid sequence revealed a significant homology with YHRO 16c/Ysc 84, the yeast Src homology 3 domain-containing protein, for which transcripts are expressed at high levels during meiosis. The sequence identity was remarkable at the amino terminus as well as the carboxyl-terminal Src homology 3 domain, suggesting that the novel protein is a mouse homolog of the yeast protein, and we have termed this protein Sh3yl1. In northern blot study, the transcripts were detected not only in the skin but also in other tissues, especially the kidney, stomach, small intestine, and colon. Furthermore, in mouse skin, the expression of these transcripts basically followed the hair-growth cycle, increasing significantly during mid and late anagen phases, and decreasing during catagen, telogen, and early anagen phases. An in situ hybridization experiment showed that the Sh3yl1 transcripts are expressed predominantly in the hair bulb, the hair shaft, inner root sheath, and outer root sheath in the lower half of the follicle during mid and late anagen phases. These transcripts were not detected in catagen, telogen, and early anagen hair follicles, or in other skin components. Thus, these data suggest the possible involvement of Sh3yl1 in the development of hair follicles during the anagen phase.