TCIRG1 and osteopetrosis: Human TCIRG1 mutations, encoding ATP6i, are associated with autosomal recessive osteopetrosis type I (OPTB1, OMIM #259700).1,4–6 We previously generated a mouse model of human osteopetrosis through Atp6i deletion (Atp6i−/− mice),7 providing a useful tool for elucidating the mechanism behind root formation defects.