The SP assay provides a powerful method to functionally define TSC and CSC by ABC drug transporter-dependent dye extrusion and complements other stem cell identification methods that are based on either antibody-mediated staining of stem cell-associated surface markers (e.g., CD24, CD34, CD44, CD90/Thy-1, CD105/endoglin, CD117/c-kit, CD133/prominin-1, CD166, CD326/EpCAM, Lgr5, and Sca-1) or detection of stem cell-related enzymatic activity (i.e., aldehyde dehydrogenase) [3, 19, 40]. The gene discussed is CD44; the disease is tuberous sclerosis.