The Sex-lethal early splicing pattern uses a default mechanism dependent on the alternative 5' splice sites.

The Sex-lethal (Sxl) early transcripts have a unique 5' exon and a splicing pattern that differs from that of the late transcripts. While the late transcripts are regulated sex specifically by control of exon 3 inclusion, the early transcripts are not. While the late transcripts include exon 3 by default, the early transcripts skip exon 3. Splicing patterns of a reporter gene that mimics the early transcript, and its variants, were analyzed in Drosophila transformants and tissue culture cells. The results demonstrate that the early, in contrast to the late, splicing pattern is not regulated by stage-specific or sex-specific trans-acting factors, and so the pattern appears to arise from some type of intrinsic splice site preference or compatibility. Inclusion or exclusion of exon 3 is determined by the identity of the upstream 5' splice site region as late or early. The important region of the early exon lies within 233 nucleotides of the immediately adjacent intron.