A mouse embryonic cDNA containing two opa-like (CAX)n repeats was isolated on the basis of its cross-hybridization with a Drosophila K10 cDNA. Such repeated sequences were present in different murine mRNAs, some of which were specifically expressed during fetal life or in different adult tissues. This suggests that, as already described for Drosophila, opa-like sequences are parts of proteins involved in ontogenic or cell-type-specific functions in vertebrates. However, unlike Drosophila, such repeated sequences were not found within the murine homeo-boxes containing genes of the Hox-1 complex.