Methaneselenol, commonly referred to as methylselenol, is an organoselenium compound that is a selenium analogue of methanol. It consists of a methyl group covalently bound to a selenol group. Methylselenol is a naturally produced metabolite of selenium (Se), an essential trace element (PMID: 25258323). Inorganic selenium is mainly represented by selenate and selenite, whereas the organic selenium forms are represented by the selenoamino acids: selenomethionine (SeMet), selenocysteine (SeCys) and selenium methylselenocysteine (MSC) (PMID: 25258323). SeMet and MSC can be found in vegetables such as garlic and onions. Selenite is reduced in the presence of glutathione to selenide (H2Se).  SeMet and selenocysteine (SeCys2) are also primarily converted to selenide and incorporated into selenoproteins or selenosugars. On the other hand, MSC (a natural metabolite) and the antineoplastic drug methylselenic acid (MSA) are converted to methylselenol by the enzyme beta-lyase (PMID: 25258323). Both selenide and methylselenol are highly reactive and both can interconvert through methylation/demethylation activities. Methylselenol is thought to be most significant in the anticancer activity of Se compounds (PMID: 30241340). Redox active Se compounds such as methylselenol have gained significant attention in the past decade as potential cancer therapeutic agents. Methylselenol is known for its action to selectively kill transformed cells through mechanisms that include increased formation of reactive oxygen species, induction of DNA damage, triggering of apoptosis, and inhibition of angiogenesis. Methylselenol also regulates the expression of ligands that trigger immune activation through the lymphocyte receptor NKG2D. The expression of NKG2D ligands is induced by stress-associated pathways that occur early during malignant transformation and enable the recognition and elimination of tumors by activating the lymphocyte receptor NKG2D (PMID: 25258323). This suggests that the application of selenium compounds that are metabolized to CH3SeH could improve NKG2D-based immune therapy for cancer treatment.