ESR1 and neoplasm: While preclinical evidence indicates that each of these four different phosphorylation events regulate ERα transcriptional activity to some degree, more limited clinical evidence suggests that their prognostic and predictive values are site-specific and not equivalent, with pS118 and pS167 tumor immunoreactivity associated with antiestrogen responsiveness while pS305 tumor immunoreactivity is associated with antiestrogen resistance [8–10, 12, 13].