My son just started Kuvan They have been given it for years in Europe It was called BH4 it was picked up by biomarion in 2004 and american trials started. In europe they were giving it to pt who had tetrabioctian (SP?)deficincies and also had pku they found that it does not help with the first but it does turn on the enzyeme that brakes phe down. So to be a responder the patient has to have some enzyemes not all pku people do it depends on the type of Pku you have Like my son has cpku but his genetic testing showed that he has a cpku gene and a mild pku gene this makes him a good canidate to be a responder. Inoreder to be a responder there has to be 30% drop in phe 1 wk after starting kuvan so far landen my son is off to a good start with a 50% reduction. It can be expensive but biomarion works with you and your insurance Biomarion also told me there are grants in place if the copays were rediculous, we are paying our max copay for scibs at 75 dollars a month not to bad.