RRM2B and amyotrophic lateral sclerosis: Examining each gene in the p53 pathway according to its p53 weighting in PROGENy revealed that the genes with the strongest responsiveness in p53 activity in ALS iPSMNs included CDKN1A, SESN1, RRM2B, MDM2, C2orf66, ZNF561 and ZMAT3 (Fig. 2e).