With hyperphe, whether he’ll have to stay on the formula will really depend on how well he processes phe. If his levels are under 10 as an adult off-diet, he probably won’t have to be on formula. The BH4, if he were to respond, might make it more likely that his levels would stay lower. And yes, I do have to take the formula, but I’m not actually hyperphe, it turns out, I actually have mild PKU, and my levels are a bit higher off-diet than those with hyperphe. To get my levels down below 10, I have to be on the diet.