Mycosis Fungoides (MF) and Sézary syndrome (SS) are the most representative entities and show a heterogeneous mutational landscape that comprises elements of the TCR signaling, NF-κB and the DNA damage/repair pathways, being copy number variations (CNV) more common than somatic mutations in the JAK/STAT pathway. The gene discussed is NFKB1; the disease is mycosis fungoides.