A new murine homeo-box (Hox1-3) has been isolated and studied with respect to its structure and transcriptional pattern. This homeo-box is part of a gene which is specifically regulated during mouse prenatal development and expressed in a restricted number of teratocarcinoma tumours as well as in adult testis. Hox1-3 is shown to be a member of a cluster of at least four homeo-boxes lying within a 75-kb segment of DNA on mouse chromosome 6. The structure of the whole cluster, the Hox-1 locus is presented.