The helix-loop-helix genes LYL, SCL and E2A are associated with chromosome translocations found in human lymphoid leukemias. To establish their hematopoietic expression patterns, we have isolated murine LYL and SCL cDNA clones and investigated the expression of all three genes by Northern blot analysis of 58 murine hemopoietic cell lines and tissues. The nucleotide sequences of LYL cDNA clones revealed alternative 5' untranslated sequences and differential splicing within the 5' portion of the coding region that may produce a LYL polypeptide lacking an N-terminal segment. The LYL gene was expressed in most myeloid, erythroid and B lymphocyte cell lines and displayed two alternative size classes of transcripts, the smaller size class (1.5-1.8 kb) being typical of the erythroid lineage and the larger class (2.0-2.3 kb) of the B cell lineage. These two size classes were found to differ in the 5' untranslated region. Thus, expression of the LYL gene appears to be differentially regulated in different hemopoietic cell types. In contrast, the E2A gene was expressed throughout the hemopoietic compartment as a single dominant transcript (3.5 kb). SCL expression was restricted to erythroid, mast and early myeloid cell lines, and the level of SCL transcripts (3.0 and 4.7 kb species) increased markedly during DMSO-induced differentiation of erythro-leukemia cells. Hence the SCL gene product may be an important regulatory factor for the erythroid lineage. The low or undetectable expression of both SCL and LYL in most T lymphoid cell sources is consistent with the view that the translocations of these genes in human T cell leukemias alter their normal regulation and may thereby contribute to neoplasia.