Acute Lymphoblastic Leukemia (ALL) results from environmentally-triggered in utero translocations between the Mixed Lineage Leukemia (MLL) gene and partner genes. In the most frequent cases of ALL, this partner gene is one of the AF4 family (AFF) of transcription factors. The newest AFF member to be discovered and cloned is AFF-4/AF5q31/MCEF. MCEF interacts with a transcription factor necessary for transcription of HIV-1. In addition, evidence suggests that male knockout mice are azoospermic. Therefore, the characterization of MCEF is clinically and theoretically important. The purpose of my research was to further characterize MCEF. In this paper, I first review the AFF members, focusing on MCEF. I then show a series of experimental results addressing MCEF isoforms and HIV-1 repression domains, as well as the generation of anti-MCEF antisera. Finally, I highlight intriguing results with live virus replication assays that suggest how MCEF could be exploited as a therapeutic target for AIDS.