We have molecularly cloned a mouse homeobox containing gene by isolating cDNA and genomic clones. The gene is located in a previously described cluster on chromosome 11 (Hart et al. (1985) Cell 43, 9–18) and was identified as the Hox2.3 gene. We present the complete mRNA. sequence of this gene and describe similarities to other homeobox containing genes, among which its human homologue, the cl gene. High expression of the Hox2.3 gene was found in kidney, testis, and spinal cord of adult mice, in the spinal cord of 12.5–17.5 day embryos and in differentiating EC cells depending on their treatment. Three different treatments of the pluripotent EC cell line P191 each leading to the induction of a specific differentiation pathway, resulted in all cases in induction of Hox2.3; however, major quantitative differences in this response were observed.