Next, to test whether specific gene editing through HDR could be generated by the CRISPR/Cas9 system, we chose exons 10 and 16 of the RET gene which harbour, respectively, the mutation causing Multiple Endocrine Neoplasia 2A (MEN2A) and Hirschsprung disease (MEN2A/HSCR: C620R in humans and C612R in chickens) and MEN2B (M918T in humans and M910T in chickens) (Supplementary Fig. 1A)26, 27. Here, RET is linked to Hirschsprung disease.