Chapter 1 3 figures for the respective free acids themselves. Of particular note in Table 2 is the very high amount of cystine that in whole hair amounts to nearly 17% of the residues, as this amino acid but can be as high as 37% in some of the hair’s subcomponents7. The table shows ½-cystine in that, two units of the actual amino acid cysteine each with a sulphydryl-containing side chain have been oxidized by post-translational modification in the hair follicle to produce disulphide cross-linked cystine. ½-cystine therefore reflects the amount of the free sulphydryl cysteine in the follicular hair proteins. It is cystine that is primarily responsible for the chemical and structural resilience of hair. Table 2. Amino acid composition of human head hair AMINO ACID Content in residues % Full name 3-Letter code 1-Letter code Cysteic acid --- --- 0.34 Aspartic acid Asp D 5.77 Threonine Thr T 7.45 Serine Ser S 11.52 Glutamic acid Glu G 12.95 Proline Pro P 6.80 Glycine Gly G 6.11 Alanine Ala A 4.62 Valine Val V 4.98 ½ - Cystine Cys C 16.73 Methionine Met M 0.21 Isoleucine Ile I 2.54 Leucine Leu L 6.49 Tyrosine Tyr Y 2.14 Phenylalanine Phe F 1.65 Lysine Lys K 2.49 Histidine His H 0.71 Tryptophan Trp W N.D. Asparagine Asn N ---- Glutamine Gln Q ----