A 59-year-old woman with persistent lower-leg erythema and pain, initially misdiagnosed as cellulitis, was later diagnosed with erythema nodosum (EN) and acute myeloid leukemia (AML), French-American-British (FAB) subtype M2, carrying a FLT3-internal tandem duplication (ITD) mutation. The gene discussed is FLT3; the disease is Erythema nodosum.