Myelencephalic blebs (my) is a recessively inherited mutation on mouse Chr 3. Embryonic subepidermal blebs cause abnormalities of the feet, eyes, skin and hair. Other abnormalities include preaxial polydactyly, syndactyly of the middle digits, acrania, pseudencephaly and renal agenesis. An insertional mutant exhibiting the same phenotype as my was produced as a by-product of a transgenic experiment and was subsequently shown to be allelic with my. Using the inserted transgene as a "molecular tag", DNA sequences flanking the transgene were isolated. The flanking sequences were mapped using the European Collaborative Interspecific Backcross and Fluorescence in situ hybridisation; both showed that the flanking DNA was on mouse Chr 5. Further studies on the "insertional mutant" indicated that the observed mutant phenotype was not associated with the transgene insertion but was probably due to a spontaneous mutation occurring at the time of the transgenic experiment. A positional cloning strategy was thus undertaken using an intersubspecific backcross between the mutant mouse and the wild-derived Mus musculus castaneus. The backcross progeny were analysed using simple sequence length polymorphisms (SSLPs) in the form of PCR primers. This backcross analysis has defined the map position of my on mouse Chr 3 relative to DNA markers. A more detailed analysis of the critical region around my will allow the isolation of Yeast Artificial Chromosomes (YACs) which span the region and facilitate the subsequent physical mapping and gene isolation.