The SV40 T-antigen gene can have two introns.

F8dl is an SV40 early-region mutant that lacks over 60% of the DNA sequences normally used to encode large T antigen. This mutant employs a novel splice donor junction at nucleotide 4425 to produce a family of doubly spliced messages. A similar splicing pattern with wild-type SV40 mRNA has been observed, indicating that the wild-type gene for T antigen can also have two introns. A single G-to-T base change at nucleotide 4425 is sufficient to eliminate this novel donor splice junction.