In contrast to classical OI caused by COL1A1 and COL1A2 variants or a mutation in the IFITM5 gene (OI type V), which result in hypermineralized bone matrix, defects in PLS3 cause significant hypomineralization of the bone matrix [31,32,33]. The gene discussed is COL1A1; the disease is osteogenesis imperfecta type 5.