Nucleotide polymorphism in the Est6 promoter, which is widespread in derived populations of Drosophila melanogaster, changes the level of Esterase 6 expressed in the male ejaculatory duct.
Odgers WA, Aquadro CF, Coppin CW, Healy MJ, Oakeshott JG. Genetics. 2002 Oct; 162(2):785-97.