Differential diagnosis of SJS-TEN involves distinguishing it from Erythema Multiforme Major (EMM), which can be identified by the appearance of targetoid lesions and through immunohistochemical tests for markers like granulysin, perforin, granzyme B, CD4, and Treg, as well as autoimmune bullous diseases, which can be ruled out using direct immunofluorescence (DIF) and ELISA. The gene discussed is PRF1; the disease is toxic epidermal necrolysis.