The exact structure of the epicuticle is unknown. Pioneering studies identified a protein layer covered by a fatty acid, later determined to be the branched chain fatty acid 18-methyl eicosanoic acid (18-MEA) covalently linked to cysteine residues near the surface via a thioester linkage . Subsequent work determined that the epicuticle layer primarily consists of ultrahigh sulfur proteins (UHSP) from the KAP family, in addition to keratin proteins and structural lipids . KAP-5 and KAP-10 have been identified as the major components of the exocuticle . Keratins and KAP proteins, in general, are insoluble and form intermediate filaments as part of an epidermal structure . There are no experimental structures available for either KAP-5 or KAP-10 families, nor any known publicly available highly homologous protein structures in the protein structure database . Asbury and Street showed that the cuticle of hair does not contain crystalline domains, in contrast to the cortex, and is not as highly organized at the molecular level .