The X-chromosomal complementation unit lethal(1)optomotor-blind [l(1)omb] is defined by lack of complementation among over a dozen recessive lethal mutations that map to the omb gene locus. Mutations in l(1)omb also fail to complement viable mutations of three seemingly unrelated functions in this region: bifid (bi), manifesting defective wings, Quadroon (Qd), a semi-dominant mutation expressing abnormal tergite pigmentation, and In(1)ombH31, giving rise to a normal external morphology but with discrete defects in the optic lobes and behavior. The locus encodes a 70-kilobase primary transcript that is spliced into a 6-kilobase mature RNA. cDNAs for this transcript were isolated and sequenced and the derived amino acid sequence was analyzed. Certain features of this sequence suggest that the l(1)omb gene product is a nuclear regulatory protein. The lethal phase of various apparent null mutants was determined and found to occur mainly in the pupal stage. A large proportion of all hemizygous mutant males develop to pharate adults that eclose only rarely but can be rescued from the pupal case. These animals show a severe maldevelopment of the optic lobes. In addition they have only rudimentary wings as well as a Quadroon-like abdominal pigmentation. Thus, in the lethal mutants those parts of the body are affected for which independent viable mutations have been previously described in the omb locus, such as optomotor-blind, bifid, and Quadroon.