Variants of IL-37. The five splice variants transcripts of the IL-37 gene are shown. Exons 5 and 6 are shared with all five isoforms. Exon 1 is absent in IL-37a only. IL-37c shares with IL-37b exons 1, 2, 5 and 6. IL-37d is similar to IL-37b, sharing exons 1, 4, 5 and 6. E, exon; IL, interleukin.