SLC7A5 and phenylketonuria: As dietary tyrosine supplementation alone does not prevent the neurocognitive features seen in individuals with PKU (Batshaw et al., 1981), it is hypothesized that elevated levels of phenylalanine in the blood compete with other amino acids for binding to the large amino acid transporter (LAT1; also known as SLC7A5) in the blood-brain barrier.