In addition to the typical type I collagen, there is evidence of a type I α1-homotrimer collagen consisting of three α1(I) chains (Jimenez et al. 1977). A third variant consisting of one α1(I), one α1(III) and one α2(I) chain, referred to as oncofetal collagen, has also been identified (Pucci-Minafra et al. 1995).

The carboxyterminal propeptide of type I procollagen (PICP) and the aminoterminal propeptide of type I procollagen (PINP) are enzymatically removed from the procollagen molecule by specific proteases. PICP and PINP are indicators of the synthesis of type I collagen. Cleavage of PICP is required for the initiation of fibril formation, whereas PINP may be retained in the collagen molecule resulting in type I pN-collagen. The retention of PINP is transient, unlike the presence of IIIpN-collagen, and has been considered to regulate fibril diameter (Fleischmajer et al. 1985).