A novel cDNA clone, A15, was isolated by the differential screening of a cDNA library of an immature T cell line, HPB-ALL using radioactive cDNA probes from the mRNA of either HPB-ALL or peripheral blood lymphocytes. It hybridized to a single mRNA species of about 2.0 kilobases which is expressed in HPB-ALL cell line, but not in the PBL or a promyelocytic leukemia cell line, HL-60. The A15 gene codes for a protein of 244 amino acids which contains four potential transmembrane domains and four possible N-linked glycosylation sites. A computer-aided comparison showed a marked similarity to several other membrane proteins: CD9, CD37, CD53, TAPA-1, Sm23, CO-029, and ME491/CD63.