A method of diagnosing Phenylketonuria (PKU) in a subject in need thereof is disclosed. The method comprises detecting phenylalanine fibrils in a tissue of the subject, wherein a presence or level above a predetermined threshold of said phenylalanine fibrils in said tissue, is indicative of PKU in the subject. Antibodies capable of detecting phenylalanine fibrils are also disclosed as well as additional uses.
