KEL encodes a zinc metalloendopeptidase with endothelin-converting enzyme activity that cleaves endothelin peptides, showing marked preference for endothelin-3 1. The protein is a type II membrane glycoprotein (Mr ~93,000 in humans) disulfide-linked to the XK protein, and its expression is regulated by erythroid transcription factors GATA1 and KLF1 binding to the KEL promoter 2. While originally characterized as erythroid-specific, KEL expression extends to brain, testis, lymphoid tissues, and follicular dendritic cells 3. In transfusion medicine, KEL is clinically significant as a polymorphic blood group antigen; mismatched KEL antigens can cause severe transfusion reactions, and maternal alloimmunization may lead to hemolytic disease of the newborn 1. Emerging evidence reveals oncogenic potential: KEL is aberrantly overexpressed in acute erythroleukemia patients, where it promotes cell proliferation, induces epigenetic changes via H3K27 acetylation, and contributes to drug resistance and immune evasion 4. Genetic variants, including novel KEL*01 and KEL*02 alleles, affect phenotypic expression and serologic detection, necessitating molecular genotyping for accurate blood typing 5. XK deficiency underlying McLeod syndrome highlights KEL's functional coupling to membrane protein complexes 1.