Salz 9630262 Gene expression can be controlled by alternative splicing of introns and one of the best studied examples of that mechanism is the master switch gene of sex determination, Sx1, in Drosophila melanogaster. The gene is transcribed in both sexes, but only in the female is a functional protein made as a consequence of alternative splicing. The female specific splice requires the function of snf, a gene that encodes a protein with homology to the human splice proteins U1A and U2B. The goals of this work are (1) to determine whether snfs role in Sx1 splicing regulation requires its U1 snRNP and/or U2 snRNP function, (2) to determine if the human U1A and/or U2B proteins function in Drosophila and (3) to identify the Snf protein domains necessary for its role in Sx1 splicing regulation. %%% As a consequence of the evolutionary conservation of the components of the RNA splicing machinery, the data generated from the proposed studies will provide a significant contribution to the understanding of mechanisms that regulate pre-mRNA splicing in many organisms, including humans.