ESR1 and breast cancer: One study that provides some of the most direct and striking evidence to support this found that in ER+ human MCF7 breast cancer cells, which lie dormant in vivo following intracardiac injection (28, 87–89), overexpression of PTHrP (1–141) pushes these cells out of quiescence, switches them to a highly osteolytic phenotype and dramatically increases tumor burden in the bone (87).