Identification and quantitation of circulating tumor cells in patients with cutaneous T-cell lymphoma -mycosis fungoides (MF)/Sézary syndrome (SS)- are required for diagnosis and precising the actual staging and response to treatment. The current flow cytometry techniques used in clinical laboratories do not correctly allow to compare results in a clinical setting. Furthermore, now we know that the phenotype of tumor cells partially overlaps with that of normal TCD4+ cells, and it is rather heterogeneous. The GENERAL OBJECTIVE of this project is to apply flow-cytometry standardized strategies for rapid, specific, sensitive, and reproducible detection and quantitation of tumor cells in patients with MF/SS. For this purpose, in the first phase of the project we will design an optimal combination of markers to detect tumor cells by spectral flow-cytometry, and then the specificity and analytical sensitivity of the new combination/procedure will be assessed in blood samples -to be later applied to skin samples-, and finally reference databases will be created for the automatic analysis of cytometry data. In a second phase of the project, the developed method will be validated in a multicenter manner, through the demonstration of its practical applicability and clinical utility (speed and precision) in blood samples (and skin, where appropriate) for diagnosis, staging, and treatment monitoring. In parallel, the tumor microenvironment (residual normal immune system) will be explored -by applying the panel designed in the first phase together with additional immune-monitoring panels by flow cytometry-, and its relationship with clinical-biological heterogeneity of the tumor will be analyzed. In the two phases of the project, cytometry data will be compared with the gold standard approach to identify tumor T cells (through the identification of clonal rearrangement by PCR and/or NGS, performed on cell populations previously sorted by flow cytometry).