In most cases, splicing removes introns as single units from precursor mRNA transcripts. However, in some cases, especially in mRNAs with very long introns, splicing happens in steps, with part of an intron removed and then the remaining intron is spliced out in a following step. This has been found first in the Ultrabithorax (Ubx) gene of the fruit fly, Drosophila melanogaster, and a few other Drosophila genes, but cases in humans have been reported as well.