 A subset of group II introns encode essential splicing proteins, known as intron-encoded proteins or IEPs, in intronic ORFs.The length of these introns can, as a result, be up to 3 kb.Splicing occurs in almost identical fashion to nuclear pre-mRNA splicing with two transesterification steps, with both also using magnesium ions to stabilize the leaving group in each step, which has led some to theorize a phylogenetic link between group II introns and the nuclear spliceosome.