This IHC-based classifier guides the differential diagnosis between ALK- ALCL and CD30high PTCL, NOS with a sensitivity of 0.86 and specificity of 0.9, suggesting pSTAT3-S727 is a promising differential biomarker for ALK- ALCL vs. CD30high PTCL, NOS. Here, ALK is linked to anaplastic large cell lymphoma.