Abstract Sex is critical to the survival and evolution of sexually reproducing organisms including mosquitoes. A dominant male-determining factor (M factor) is the primary signal that controls sex-determination in mosquitoes. Nix, the M factor in the yellow fever mosquito Aedes aegypti, is the first M factor found in mosquitoes. In a simplified model, the expression of Nix, a predicted RNA-binding protein, leads to male- specific splicing of the pre-mRNAs of two conserved transcription factors, doublesex (dsx) and fruitless (fru), which program male sexual differentiation. In Aedes the M factor is located within the male- determining locus (M locus) on one of the “autosomes”. This pair of “autosomes” are so-called homomorphic sex chromosomes that are cytologically indistinguishable except in the region around the sex locus. The Ae. aegypti M locus is a ~1.3 Mbp repeat-rich region that contains Nix and four other protein-coding and 25 long non-coding RNA genes. The Nix transgene alone, in the absence of the M locus, is sufficient to convert females into fertile albeit flightless males, and myo-sex, a myosin heavy chain gene also in the M-locus, is required for male flight. The M- and m-bearing chromosomes in Aedes mosquitoes provide an opportunity to gain insights into the evolution of homomorphic sex chromosomes. In addition to its basic biological importance, Ae. aegypti is a major vector for the dengue, chikungunya, and Zika viruses. No specific treatment for dengue exists and the first dengue vaccine is recommended only for a limited population. Prevention of these vector-borne infectious diseases relies heavily on effective vector control. However, increasing insecticide-resistance poses a significant threat. Therefore, novel control strategies are urgently needed. Only female mosquitoes feed on blood and transmit pathogens, and for the most part females determine the size and distribution of the mosquito population. We are interested in deciphering the mechanism of sex-determination, investigating sex chromosome evolution, and translating such fundamental knowledge into safe, efficient, and diverse methods to control diseases that are transmitted by Ae. aegypti. Building on recent progress, we will pursue the following specific aims: 1) Decipher the sex locus in Ae. aegypti, 2) Identify and characterize the target(s) of Nix, and 3) Develop efficient sex-separation methods through Nix-mediated innovations.