MXI1 encodes a transcriptional repressor that functions as a tumor suppressor by antagonizing MYC oncogene activity 1. The protein dimerizes with MAX to form sequence-specific DNA-binding complexes that recognize core sequences and compete with MYC-MAX dimers for binding sites, thereby repressing MYC-mediated transcription 1. MXI1 is frequently inactivated in human cancers through allelic loss and mutations. In prostate cancer, 53% of tumors demonstrate loss of MXI1 alleles, with many mutations rendering the protein unable to bind DNA in association with MAX 2. Similarly, 7 of 11 glioblastomas showed MXI1 allelic loss 3. Functional studies demonstrate that MXI1 restoration suppresses cell growth and causes G2/M cell cycle arrest in glioblastoma cells lacking endogenous expression 3. The MXI1 gene spans approximately 60 kb on chromosome 10-q25 and comprises six exons 4. In lung cancer, MXI1 acts through a complex regulatory network involving microRNA-300 and KLF9 to suppress tumor progression and immune evasion 5. These findings establish MXI1 as an important tumor suppressor that maintains cellular homeostasis by counteracting oncogenic MYC signaling.