PPP1R15A and medulloblastoma: Importantly, the level of CHOP was strongly increased in medulloblastoma in Ptch1+/−; Gadd34−/− mice compared to Ptch1+/−; Gadd34+/+ mice (Figure 2C, 2D), suggesting that GADD34 homozygous mutation strongly elevates the ISR in medulloblastoma in Ptch1+/− mice.