Erythroblast transformation-specific domain (ETS) transcription factors regulate some of the critical molecular mechanisms controlling the differentiation of multipotent haematopoietic progenitor cells into effector B-lymphocytes. The SPI-group ETS-protein transcription factors PU.1 and SPI-B play essential and, although coexpressed and binding to similar DNA sequences, unique roles in B-cell differentiation in mice. Mouse SPI-C is an SPI-group ETS protein expressed temporarily during B-cell development and in macrophages. Here we present the genomic organization of the mouse SPI-C gene, and show by rapid amplification of cDNA ends (5'-RACE) analysis that transcription of the mouse SPI-C mRNA starts at a single site producing a single processed transcript. We have also isolated a cDNA clone encoding the human SPI-C homologue, which displays 65% amino acid identity to the murine protein. In addition, we show that the genomic structure of the human and mouse genes are similar, containing a 5' non-coding exon followed by five coding exons. Human SPI-C mRNA is preferentially detected in foetal and adult spleen, lymph nodes and at lower levels in bone marrow and foetal liver. Finally a phylogenetic prediction analysis of SPI-group protein sequences suggest that the SPI-C proteins form a distinct subgroup, with human SPI-C being closest related to the mouse SPI-C protein.