In  Drosophila,  the  ratio of X
   chromosomes  to  sets  of autosomes (X:A) affects transcription of the
   Sex lethal (Sxl) gene.

   In  females,  the  Sxl  gene  product  (SXL)  binds  to premRNA of the
   transformer  (tra)  gene,  and  directs  its  splicing  to  produce  a
   female-specific  TRA protein, which combines with the TRA-2 protein to
   direct  female-specific  splicing  of  dsx  pre-mRNA,  resulting  in a
   female-specific protein (DSX-F).

   In  males, the absence of SXL results is a male-specific processing of
   tra pre-mRNA, producing no functional TRA protein, which in turn leads
   to  male-specific  processing  of  dsx transcripts and a male-specific
   (DSX-M)   protein   that   activates   the  pathway  for  male  sexual
   development.