Type I collagen (Col-I), a fibrillary type collagen, is the most abundant collagen in the human body accounting for 90% of all collagen. It is the major protein component of the bone extracellular matrix and is commonly present in tissues where increased tensile strength is required, such as ligaments, tendons, menisci, skin, blood vessels, intervertebral discs, and cornea. The dominant isoform of type 1 collagen is a heterotrimer (i.e., a triple-helix structure) consisting of two pro-alpha1(I) chains and a pro-alpha2(I) chain. However, homotrimers of three pro-alpha1(I) chains, known to play a key role in wound healing, have been documented in fetal tissues, tumors, and some fibrotic lesions. In humans, pro-alpha1(I) and pro-alpha2(I) chains are transcribed and processed from COL1A1 and COL1A2 genes, respectively. The majority of type I collagen mutations result in bone and connective tissue disorders, in particular, osteogenesis imperfecta types I-IV (brittle bone disease), Ehlers-Danlos syndrome classical type, Ehlers-Danlos syndrome type VIIA, idiopathic osteoporosis, and Caffey disease. Biomarkers for type I collagen are divided into two categories, degradation, and synthesis biomarkers. Col-I degradation biomarkers include Col-I neoepitope (C1M), C-terminal telopeptide of Col-I (CTX-I), and Col-I-derived crosslinked carboxy-terminal telopeptide (ICTP). Col-I synthesis biomarkers include carboxy-terminal propeptides of pro Col-I (PICP) and amino-terminal propeptides of pro Col-I (PINP).

Type I collagen has been visualized using a variety of techniques. Scanning and transmission electron microscopy provide high-resolution visualization of individual collagen fibers, but, is labor-intensive and expensive to perform. Western blot, IHC, IF and ELISA using type I collagen specific antibodies are more cost-effective alternatives, providing similar levels of sensitivity and are easily reproducible. In addition, single-cell RNA-sequencing analysis has been used to identify collagen I-producing cells and lineages in bone and bone marrow fractions.