Expression of the gene for hematopoietic cell specific protein is not restricted to cells of hematopoietic origin.

Hematopoietic cell line specific protein (HS1) is an intracellular signaling protein that has been reported as specifically expressed in hematopoietic cells. HS1 is known as a major substrate of protein-tyrosine kinases following activation by B-cell or T-cell receptor complexes. We report the first evidence that HS1 is also expressed in a variety of tissues different from hematopoietic tissues by using sensitive expression analysis including real-time quantitative RT-PCR. While former studies on HS1-expression were mostly limited to cells of hematopoietic origin, we screened a larger number of human tissues including tumor samples. Normal lung tissue showed a high degree of HS1 expression, second to the expression of hematopoietic cells. Expression of HS1 in tumor tissues was also clearly detectable. Our findings suggest that the signaling protein HS1 is involved in pathways different from the ones that have a specific role in the intracellular processes of hematopoietic cells.