Type I endometrial cancer accounts for 65–70% of cases and is associated with grade 1–2 endometrioid histology, younger age of onset, retention of estrogen receptor (ER) and progesterone receptor (PR) status, a history of unopposed estrogen, and deletions in k-Ras, PTEN, or mismatch repair mechanisms (Bokhman, 1983, Matthews et al., 1997). The gene discussed is ESR1; the disease is endometrial cancer.