A recent study performed with large-cell transformed MF (a potentially more aggressive variant of MF) patients indicated that reduced CD26/DPP4 expression on CD4+ T cells might be associated with poor prognosis [139], whereas a retrospective cohort analysis of 11 patients with erythrodermic CTCL indicated that the frequency of CD26−/CD4+ T cells was not a reliable marker of treatment response or disease progression [140]. This evidence concerns the gene DPP4 and mycosis fungoides.