2010-07-152010-07-232017-04-072010-07-152010-07-232017-04-072009-122010-07-14http://hdl.handle.net/1969.1/ETD-TAMU-2009-12-7419In Drosophila, XX embryos express Sxl from the early promoter, SxlPe, and become females. At the same time, XY embryos with only one X chromosome become males. I investigated the role of repression in the establishment of the strict regulation of SxlPe. I found that the co-repressor Groucho, is responsible for amplification of the two-fold difference in X-encoded activator genes into an all-or-nothing difference in Sxl expression. Three new basic helix-loop-helix repressors of Sxl were identified: Hey, Cwo and the prooncogene Myc, all of which are maternally supplied. I have shown that Myc specific repression is important as early as cycle 10, which is 2 cycles earlier than the onset of normal Sxl expression.engDrosophilapromoterrepressorGrouchoSxlHeyCwoMyc.Book