Diagnosis requires clinical erythroderma and evidence of peripheral blood involvement, including >1000 Sézary cells/μL, an expanded CD4+ T-cell population with a CD4:CD8 ratio >10:1, or an expanded T-cell population with an abnormal immunophenotype including >40% loss of CD7 or >30% loss of CD26.1 The gene discussed is CD7; the disease is exfoliative dermatitis.