The homeobox gene Mbxis thought to play important roles in the development of eyes and tectum in zebrafish. We isolated mouse Mbx cDNA and analyzed its expression pattern during early mouse embryogenesis. Expression is predominantly restricted to the midbrain region at E9.5. At subsequent stages of development, Mbx transcripts were also found in the forebrain in addition to midbrain. Thus, the Mbx gene provides a useful molecular marker for early mouse midbrain development and may play a critical role in brain development.