The presence of homoeo box sequences in the genomes of vertebrates has suggested that these metazoans possess the equivalent of the homoeotic genes that have a key role in regulating the development of the fruitfly Drosophila melanogaster. We report here that a novel murine homoeo box-containing gene is expressed in embryonal carcinoma stem cells. Transcripts of the sequences that flank the homoeo box of this gene are found in these cells before and after induced differentiation, whereas a specific transcript that seems to contain the homoeo sequence is only present after differentiation.