CCL17 and adult T-cell leukemia/lymphoma: Further underlining the importance of the CCL17/CCL22/CCR4 signaling axis in ATLL pathogenesis, somatic gain of function (GOF) mutations in CCR4, detected in 26 to 29% of ATLL cases [102,103], result in increased cell migration toward CCL17 and CCL22, together with impaired CCR4 internalization [102].