The homeobox is a 180-bp DNA sequence that encodes a DNA-binding domain known as a homeodomain. To identify additional homeobox genes, Matsui et al. (1993) performed PCR using genomic DNA and degenerate primers based on conserved regions of the homeodomain. They isolated fragments of 2 novel homeobox genes, GBX1 and GBX2). The amino acid sequence of the predicted GBX1 and GBX2 homeodomains were identical. Sequence comparisons indicated that the GBX1/GBX2 homeodomain was most similar to that of chicken CHox-7. Northern blot analysis revealed that GBX1 was expressed as a 2.2-kb mRNA in 2 hematopoietic cell lines. Matsui et al. (1993) reported that the GBX1 gene is the human homolog of the mouse MMoxB, or Gbx1, gene identified by Murtha et al. (1991).