We have assessed the state of rearrangement and expression of B-and T-cell antigen receptor genes in cells of Abelson murine leukemia virus-transformed thymomas and other tumors. We found that unrearranged TcR'y genes are expressed, as are unrearranged C,I genes, in pre-T, pre-B, and myeloid cells. We also found TcR-y genes rearranged and expressed in putative pre-T cells and in cells apparently committed to the B-cell lineage. This is in contrast to the data from more mature T-and B-cell tumors. We conclude that in immature lymphoid cells both immunoglobulin and TcR-y genes are accessible for rearrangement. We discuss the implications of these observations for an understanding of the B-T lymphoid differentiation event.B and T lymphoid cells are thought to arise from a common precursor cell, although neither this progenitor nor the developmental point of divergence of the two lineages has been clearly defined (13). Abelson murine leukemia virus (A-MuLV) transforms immature cells of both lineages (8,34). Here we report the use of such transformants to investigate the relationship between developing B and T cells.The major markers of the lymphoid lineages are their antigen receptors. Three gene loci have been described for each antigen receptor complex. For B cells these encode immunoglobulin heavy chain and the K and A light chains, and for T cells they encode a, fi and y polypeptides. Although the role of y has been unclear, it will be referred to here as TcR-y, since the weight of evidence suggests it may be a component of a separate T-cell receptor (TcR) (4, 6). In particular the -y locus shares many featui es with those of the other receptor genes. Their sequences are homologous, and their gene organization is similar, especially in the need for gene rearrangement to juxtapose variable (V) region sequences close to constant regions to allow expression (reviewed (in reference 25a).For B-cell development these rearrangements have been shown to occur as an ordered series of steps. The first event is transcription from the unrearranged C,u gene, which has been taken to indicate an "opening" of this locus, a necessary condition for subsequent rearrangement of the gene. It is noteworthy that this C,u transcription occurs not only in pre-B cells but also in myeloid and T cells (23). The second step, juxtaposition of a diversity (D) region to a joining (J) region of the heavy-chain locus, has been observed in both pre-B and T cells (14,26) While examining receptor gene rearrangement and expression in a series of cloned A-MuLV-induced thymoma cell lines, we observed that TcR-y was often transcribed from unrearranged genes. This was found to occur not only in transformed immature thymocytes but also in myeloid and pre-B cells. This pattern of expression is remarkably like that of unrearranged immunoglobulin p, (Ig,u) genes. Moreover, we observed rearrangement and transcription of apparently legitimate TcR-y genes in cells which also bear immunoglobulin rearrangements thought to be absolutely specific to the B lineage....