We report a young adult with genetically and functionally confirmed CTLA-4 haploinsufficiency who developed a triad of BCR-ABL1–positive chronic myeloid leukemia (CML), bullous pemphigoid, and PLA2R-positive membranous glomerulonephritis (MGN), highlighting diagnostic and management lessons across immunology, hematology, dermatology, and nephrology. The gene discussed is PLA2R1; the disease is idiopathic membranous glomerulonephritis.