Split genes are first observed in eukaryotes. No split genes are reported yet in prokaryotes. Prokaryotic genes are contiguous. Eukaryotic genes, in contrast, contain segments of DNA that are expressed (called exons) interrupted by segments that are not expressed (called introns). During gene expression, the resulting mRNA contains both exons and introns. It then undergoes splicing by which introns are removed so that it would only contain exon sequences before it is transported to the cytoplasm.