As HDACs have been shown to be frequently deregulated in melanomas and can modulate the expression of PD-1/PD-L1 and other genes linked to immune evasion [43,44,45,46,47,48], a significant knowledge gap exists between HDAC isoforms and the T-cell inflamed TME signature in terms of predicting the therapeutic responses of ICIs. The gene discussed is HDAC9; the disease is melanoma.