105B potently degrades BET proteins in U2OS osteosarcoma cell lines (BRD4 DC50 = 0.130 nM, Dmax = 78%) and KYSE180 esophageal squamous cell carcinoma cell lines (DC50 = 40 nM, Dmax = 70%), but shows no degradation in non-cancerous, MAGEA11-deficient HEK293T cells. Here, MAGEA11 is linked to esophageal squamous cell carcinoma.