The first, which accounts for the most severe forms, is caused by biallelic mutations in TCIRG1, CLCN7, OSTM1, SNX10, and PLEKHM1 genes, encoding for proteins involved in the acidification of the resorption lacunae and/or in vesicular transport and loss-of-function mutations leading to osteoclast-rich osteopetrosis. Here, CLCN7 is linked to osteopetrosis.