Tyrosine can be thought of essential when phenylalanine is absent from diet, given that Phe is the direct precursor of Tyr. Arginine is made by mammalian cells, but not in adequate amounts to satisfy demands during growth; histidine may also be required in growing children. In addition, infants may require dietary supplementation for tyrosine and cysteine. Extra methionine may be required as well if the diet is deficient in cysteine.