The Ubx gene of Drosophila normally produces several forms of Ubx proteins through alternative splicing of two microexons. We describe here two new viable Ubx mutations that show similar and almost wild-type adult phenotypes. Molecular characterization has shown that one of them, UbxMX17, is an inversion within the Ubx transcription unit including one of the microexons involved in alternative splicing. This results in mutant flies possessing a very abnormal array of Ubx proteins, probably including spliced forms not present in wild-type flies. Yet these protein products successfully substitute for the normal ones and allow virtually normal Ubx function. We argue that the different Ubx proteins are developmentally equivalent and that the slight mutant phenotype observed in UbxMX17 flies is not due to the abnormal set of Ubx proteins but to a breakpoint in a cis-regulatory region.