In our patients with an initial diagnosis of MF that progressed to PCGDTCL with nodulo-tumoral lesions, we compared the histological and immunophenotypic features of the first indolent phase with those of the subsequent aggressive progression and, although adequate specimens were available for only two cases, documented a complete immunophenotypic shift with the loss of CD4 or CD8 expression, the expansion of the TCRδ+ population or TCR silencing, and the expression of TIA1 and granzyme B. The gene discussed is CD4; the disease is mycosis fungoides.