GJA1 and Osteopenia: Connexin43 (Cx43) is the most abundant connexin in osteocytes, and global knockout of Cx43 expression is lethal at birth.19 Conditional knockout of Cx43 in osteoblasts and osteocytes in mice led to various degrees of osteopenia depending on the differentiation state in which the deletion was induced,20,21 suggesting that functional Cx43 in osteoblasts and osteocytes is essential for normal bone mass acquisition and maintenance.