This is the solution humans and other higher vertebrates have employed. A second option would be to heighten the activity of the single X chromosome in males. This is the route flies have taken. Sex lethal , the splicing factor that directs all sex determination in the fly, regulates the function of MSLs by regulating the splicing of pre-messenger RNA of male specific lethals. The msl2 produces two transcripts that differ by an intron of 133 nucleotides in the 5' untranslated region.
Most female transcripts retain the intron, whereas most male transcripts remove it.
Unlike the previously described cases of SXL reglation, this sex-specific intron does not affect the mRNA open reading frame. The action of these male specific transcription factors is very similar to proteins of the trithorax complex . Development of an antiserum able to distinguish acetylation of lysine residues on histone H4 has provided the best clues as to a mechanism. Examination of the giant polytene chromosomes of larval salivary glands reveals that H4 molecules, acetylated at lysines 5 and 8, are distributed in islands throughout the euchromatic chromosome arms. H4 acetylated at lysine 16 is found at numerous sites along the transcriptionally hyperactive X chromosome in male larvae, but not in male autosomes or any chomosomes in female cells.