PTHLH and neoplasm: When we assessed p21 protein expression, we found inconsistent staining patterns between in vitro cultured cells and in vivo tumor sections; however, we did see a modest increase in p21 staining in tumors expressing full-length secreted PTHrP, suggesting p21 may be regulated downsteam of the intact PTHrP molecule in the context of the tumor microenvironment (Supplementary Fig. 3A&B).