The leucines are primarily the four isomeric amino acids: leucine, isoleucine, tert-leucine (terleucine, pseudoleucine) and norleucine.[1] Being compared with the four butanols, they could be classified as butyl-substituted glycines; they represent all four possible variations.
Leucine and isoleucine belong to the proteinogenic amino acids; the others are non-natural.