Additional Class I bromodomains, PCAF, CECR2, and GCN5, have also been targeted for diseases such as HIV and glioblastoma, breast cancer, and osteoporosis, respectively (Figure 1B).10–13 Inhibitor development against class I bromodomains is limited by fewer standardized assays to study compound engagement in cells. This evidence concerns the gene KAT2B and osteoporosis.