PHOX2B and Hirschsprung disease: To further clarify the implications of CALN1 and CAPN9 in HSCR, we mapped the RET, CALN1 and CAPN9, combined with some known HSCR pathogenic genes, including, ECE1, EDN3, EDNRB, GDNF, NRTN, SOX10, PHOX2B, and KIAA1279, to protein–protein interaction (PPI) network curated in STRING database (Szklarczyk et al., 2015).