45Recently, the "omnigenic" model of the genetic architecture of complex traits proposed two 46 general categories of causal genes, core and peripheral. Core genes are hypothesized to play a 47 direct role in regulating disease; thus, their identification has the potential to reveal critical 48 regulators and novel therapeutic targets. Here, we sought to identify genes with "core-like" 49 characteristics for bone mineral density (BMD), one of the most significant predictors of 50 osteoporotic fracture. This was accomplished by analyzing genome-wide association study 51 (GWAS) data through the lens of a cell-type and timepoint-specific gene co-expression network 52 for mineralizing osteoblasts. We identified a single co-expression network module that was 53 enriched for genes implicated by GWAS and partitioned BMD heritability, correlated with in vitro 54 osteoblast mineralization, and enriched for genes, which when mutated in humans or mice, led 55 to a skeletal phenotype. Further characterization of this module identified four novel genes 56 (B4GALNT3, CADM1, DOCK9, and GPR133) located within BMD GWAS loci with colocalizing 57 expression quantitative trait loci (eQTL) and altered BMD in mouse knockouts, suggesting they 58 are causal genetic drivers of BMD in humans. Our network-based approach identified a "core" 59 module for BMD and provides a resource for expanding our understanding of the genetics of 60 bone mass. 61 62 63 64 65 66 67 68 69 70 3