310Background: Telomerase reverse transcriptase (TERT) is the most frequently altered gene in urothelial cancer (UC), detected across all grades and stages of disease. We sought to characterize TERT alterations within a prospective cohort of UC treated at our institute and compare the frequency of genomic alterations in TERT promoter mutant vs wild-type UC specimens. Methods: Patients diagnosed with bladder urothelial tumors were enrolled onto an institutional review board approved prospective sequencing protocol. Tumor and matched germline DNA were analyzed for somatic point mutations, truncations, copy number alterations, and insertions/deletions using the MSK-IMPACT NGS assay that detects alterations in all exons and select introns of 410 oncogenes and tumor suppressor genes as well as the entire TERT promoter region. Results: 329 UC were sequenced of which 236 (71.7%) harbored TERT mutations, the majority being promoter region hotspots (chr5: 1295228 G > A [81%] and chr5:1295250 G > A [16%]). Patients...