This is a major challenge in CTCL, because the cell type of origin (e.g., effector memory T cells (TEM), central memory T cells (TCM), tissue-resident memory T cells (TRM))24, expression of T cell markers (e.g., CD2, CD3, CD4, CD5, CD7, CD25, FoxP3), and clonality25 can be shared by both cell types. This evidence concerns the gene CD5 and primary cutaneous T-cell non-Hodgkin lymphoma.