Whilst the majority of Endometrial Cancers (ECs) are common Type 1 endometrioid cancers, accounting for 75-80% of cases, with a good prognosis, Type 2 ECs include high-grade, clinically aggressive histologies, with poor response rates to hormonal therapies. Serous endometrial carcinoma is the second most common type, accounting for ~10 percent of cases, most with a p53 abnormality and a lesser proportion with HER2 overexpression/amplification/mutation. Clear cell endometrial carcinoma accounts for Citation Format: Clare L. Scott, Ratana Lim, Amandine Carmagnac, Cassandra Vandenberg, Gayanie Ratnayake, Genevieve Dall, Joshua Tram, Justin Bedo, Jocelyn Penington, Joep Vissers, Sean Grimmond, Matthew Wakefield, Anthony Papenfuss, Holly Barker. Building the infrastructure required to research novel therapies for 35% of women with endometrial cancer who have rare subtypes [abstract]. In: Proceedings of the AACR Virtual Special Conference: Endometrial Cancer: New Biology Driving Research and Treatment; 2020 Nov 9-10. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(3_Suppl):Abstract nr PO026.