LRRK1 and osteopetrosis: The processes of bone formation and bone resorption are regulated by systemic hormones, nutrition, local growth factors, and mechanical stimuli.1,2 A high-throughput screen aimed at the identification of the functions of over 4 500 genes led to the discovery of a leucine-rich repeat kinase 1 (LRRK1) as a critical regulator of osteoclast function and bone resorption with little effect on bone formation.3,4 The severe osteopetrosis phenotype in long and axial bones observed in Lrrk1 knockout (KO) mice makes LRRK1 an ideal drug target for the prevention and treatment of osteoporotic fractures.