• Skin-homing of neoplastic T cells, especially in advanced-stage MF and SS (Guistiniani et al., de Masson et al., Geller et al.).• Proposed as a diagnostic and therapeutic biomarker (Geller et al.).• Targetable by monoclonal antibodies; potential for reduced immune-related adverse events compared to anti-CCR4 therapy (Silva et al., de Masson et al.). This evidence concerns the gene CCR4 and synovial sarcoma.