Gain of chromosome 21 is seen in 8.5% of cutaneous T-cell lymphoma (CTCL) [141, 142], and its role in CTCL development has been assessed in vivo in cooperation with JAK3 activating mutations, through establishment of a trisomic murine model of CTCL [95]. This evidence concerns the gene JAK3 and primary cutaneous T-cell non-Hodgkin lymphoma.