NIPBL and Cornelia de Lange syndrome: For this reason, we studied the two main splice variants of the NIPBL gene, isoforms A and B, in adult and fetal human tissues, and in two patients with CdLS who were harbouring a pathological variant specific for isoform A. Isoforms A and B differ only at the carboxy-terminal end, and thus, isoform A encodes the complete protein, whereas isoform B lacks exon 47 and ends in an expanded form of exon 46 [4].