Monoallelic inactivation of CCCTC-binding factor (CTCF) in human cancer drives altered methylated genomic states, altered CTCF occupancy at promoter and enhancer regions, and deregulated global gene expression. In patients with T cell acute lymphoblastic leukemia (T-ALL), we find that acquired monoallelic CTCF-inactivating events drive subtle and local genomic effects in nearly half of t(5; 14) (q35; q32.2) rearranged patients, especially when CTCF-binding sites are preserved in between the BCL11B enhancer and the TLX3 oncogene. These solitary intervening sites insulate TLX3 from the enhancer by inducing competitive looping to multiple binding sites near the TLX3 promoter. Reduced CTCF levels or deletion of the intervening CTCF site abrogates enhancer insulation by weakening competitive looping while favoring TLX3 promoter to BCL11B enhancer looping, which elevates oncogene expression levels and leukemia burden.Keywords: BCL11B; CP: Cancer; CTCF mutation; T cell acute lymphoblastic leukemia; TLX3; competitive chromatin looping; enhancer insulation; oncogene regulation.