The loss of cut activity in Drosophila results in the transformation of the neurons and support cells of external sensory (es) organs into those of chordotonal (ch) organs. The cut locus encodes a homeo domain-containing protein, which is expressed in the cells of es, but not in ch, organs. We show by Western analyses the presence of two embryonic protein species whose approximate relative molecular masses of 280 and 320 kD are compatible with that predicted from the primary sequence. We also describe the development of the Cut protein expression pattern and show that Cut is expressed in sensory precursor cells that divide to give rise to es organs. Finally, we analyze the changes in the Cut expression pattern of several mutant alleles of the complex cut locus and show that the mutations affecting es organ development are associated with either altered protein distribution in the PNS or incorrect subcellular Cut protein localization.