Model of the three classes of 3Hyp substrate sites identified in A-clade collagen. 3Hyp always occurs in the sequence Gly-3Hyp-4Hyp, A. We propose the Class 1 3Hyp (Pro986) functions in collagen fibril molecular packing, in which the subunits are in-register dimers staggered axially by D-periods, B. D-periodic spacing is evident between the Class 2 3Hyp residues (Pro470, Pro707 and Pro944) in the triple helix, C. Occurrence of the Class 3 3Hyp residues in the (GPP)n motif exhibited clear tissue specificity, with the modification occurring almost exclusively in tendon.