You might want to look at this: http://www.kuvan.com/AboutKuvan/AboutKuvan.aspx Briefly, Kuvan is a synthetic form of BH4 (tetrahydrobiopterin), a cofactor which assists in the breaking down of phenylalanine by the enzyme phenylhydroxylase (PAH). People with PKU have reduced production of PAH. Because of how Kuvan works, those with the most residual enzyme activity (meaning that they produce more PAH and have some ability to break down phe) are the most likely to respond well to Kuvan. Generally, those with less severe forms of PKU respond better, though there are those with classic PKU that also respond. In my case, I have moderate, or variant, PKU (vPKU), and have some residual enzyme activity. I currently take 19 pills a day (dosage is based on weight) and 1 1/2 servings of formula and eat 45 grams of protein in food each day. My levels have been at around 6 on this regimen.