Insects exhibit a variety of sex determining mechanisms including male or female heterogamety and haplodiploidy. The primary signal that starts sex determination is processed by a cascade of genes ending with the conserved switch doublesex that controls sexual differentiation. Transformer is the doublesex splicing regulator and has been found in all examined insects, indicating its ancestral function as a sex-determining gene. Despite this conserved function, the variation in transformer nucleotide sequence, amino acid composition and protein structure can accommodate a multitude of upstream sex determining signals. Transformer regulation of doublesex and its taxonomic distribution indicate that the doublesex-transformer axis is conserved among all insects and that transformer is the key gene around which variation in sex determining mechanisms has evolved.