Evolution of vasopressin and oxytocin family of nonapeptides. The letter 'D' within the circle represents a gene duplication event. The numbers within the circles denote the position of the amino acid that has been substituted (shown above the circle). OT, oxytocin; MT, mesotocin; IT, isotocin; VP, vasopressin; [Lys8]VP, lysipressin, VT, vasotocin; ASV, asvatocin; PAI, phasitocin. Protein sequencing has shown that some lungfishes contain mesotocin [9, 10] and some marsupials contain [Phe2]vasopressin . They are not shown for the sake of clarity.