SPP1 and osteoporosis: We further investigated osteoblast lineage clusters in the osteoporosis data (Figure 3A). First, based on osteogenic markers, we further annotated C4 as primitive osteogenic bmMSCs, which expressed low levels of osteogenic markers; C1, C2, C6, and C15 as premature osteoblasts, which expressed ALPL and low levels of SPP1 and IBSP; and C9 as mature osteoblasts, which expressed RUNX2, SPP1 and IBSP (Figure 3B) (25).