TLR9 and melanoma: mtDNA released into the cytoplasm can activate inflammatory responses through multiple signaling pathways, including the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING), toll-like receptor 9 (TLR9), nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) and absent in melanoma (AIM2) inflammasome signaling pathways [36].