Phenotypic expression of mutations within the leg-aristal-wing complex (lawc) locus involves alteration of the number and location of macrochaetae. In Drosophila, development of macrochaetae depends on products of the two proneural genes, achaete (ac) and scute (sc), constituents of the approximately 90-kb ac-sc gene complex (AS-C). Untranscribed regions of AS-C involve enhancer-like cis-elements that are responsible for the local combination of some factors specifically determining expression of the ac-sc genes in larval imaginal discs of Drosophila. In the present work, the frequency of appearance of additional macrochaetae was tested to analyze phenotypic exhibition of the temperature-sensitive lawcp1 allele. This mutation was also genetically analyzed in combination with various AS-C alleles involving alterations of transcribed or cis-regulatory regions of this gene complex. A hypothesis that the lawc gene encodes a factor determining the specific expression of the ac-sc genes is discussed.