NOS2 and melanoma: Nitric oxide is a free radical that is largely synthesized by the NO synthase (NOS) enzyme, which exists in three established isoforms: endothelial NOS (eNOS, NOS III) and neuronal NOS (nNOS, NOS I), which are both constitutively expressed and inducible NOS (iNOS, NOS II) which is regulated at the transcriptional level by a variety of mediators (such as interferon regulatory factor-1 [191,192], NF-kB [193,194], TNF-α and INF-γ [195,196] and has been found to be frequently expressed in melanoma [197-200].