 The PCR fragments that corresponded to Protocadherins were found in vertebrate and invertebrate species.This prevalence in a wide range of species suggested that the fragments were part of an ancient cadherin and were thus termed "Protocadherins" as the "first cadherins".Of the approximately 70 Pcdh genes identified in mammalian genomes, over 50 are located in tightly linked gene clusters on the same chromosome.