In addition, sequence variants in pkrs genes are also associated with congenital diseases: a mutation in the pkr2 gene can lead to Kallmann syndrome, while sequence variations in pkr1 and pkr2 are associated with Hirschprung syndrome (HSCR), a syndrome characterized by the absence of enteric neural crest cells (NCC) in the distal part of the colon [10,22] (Table 1). Here, PROKR2 is linked to Kallmann syndrome.