We have previously cloned a segment of a gene, ABR, homologous to the BCR gene, which encodes a protein consisting of three distinct functional domains. In the present study, genomic ABR sequences were used to isolate human ABR cDNAs. Surprisingly, the two types of ABR cDNAs identified differed only in their most 5' coding sequences. These are predicted to encode proteins of 93.5 and 92.3 kDa molecular mass. ABR showed a differential expression pattern in various mouse tissues, analogous to that of BCR, and the highest level was found in brain. Similar to BCR, ABR contains a region with homology to DBL, vav, and CDC24, which are likely to or have been shown to encode GTP exchange factors. A domain of ABR with similarity to GAPrho was expressed as a fusion protein in Escherichia coli and was shown to have GAP activity toward rac. Although both ABR and BCR have GAP activity, ABR lacks homology to the serine/threonine kinase domain of BCR. Therefore, ABR is likely to have cellular functions overlapping with but also distinct from those of BCR.