Tumor cells were differentiated from non-malignant cells of the tumor microenvironment (TME) using the following five criteria: (i) negligible SMARCB1 expression, (ii) CD2 positivity (as a marker of the mature T-cell origin of PTCL); (iii) EZH2 positivity (frequently overexpressed in PTCL-NOS36, (iv) KIT positivity (based on our previous observations) and (v) high proliferative activity as exemplified by strong expression of MKI67 (Suppl. The gene discussed is CD2; the disease is mature T-cell and NK-cell non-Hodgkin lymphoma.