S1PR1 and osteopetrosis: S1PR2 is also expressed in osteoblasts and osteoclasts.210 In response to the administration of S1P, S1PR2 coordinates with S1PR1 to regulate the dynamic migration of osteoclast progenitor cells (OCPs), where S1PR1 directs positive chemotaxis toward S1P and S1PR2 mediates negative chemotaxis.211 Therefore, mice with a S1PR1 gene deletion present osteoporosis,212 but mice with a S1PR2 gene deletion present moderate osteopetrosis.211