Almost all of the chlorine-containing gas emitted from natural sources is methyl chloride (CH 3 Cl), which contributes to the destruction of the stratospheric ozone layer. Methyl chloride (CH 3 Cl) is the most abundant halohydrocarbon emitted into the atmosphere and constitutes about 17% of the chlorine currently in the stratosphere (1). CH 3 Cl is derived mainly from natural sources and contributes to the destruction of the stratospheric ozone layer. As the total abundance of ozone-depleting gases such as chlorofluorocarbons in the atmosphere has begun to decrease in recent years as a result of The Montreal Protocol on Substances That Deplete the Ozone Layer, the impact of CH 3 Cl emission from natural sources will become greater on the atmospheric chemistry. CH 3 Cl emission into the atmosphere has been estimated at 1,700 -13,600 Gg/year (1), which underscores the great uncertainty of the estimation. Oceans (2), biomass burning (3), woodrotting fungi, and coastal salt marshes (4) are the major sources of CH 3 Cl production. Recently, it was reported that large amounts of CH 3 Cl are emitted from tropical and subtropical plants, which are hence considered as the major sources of CH 3 Cl (5-7). It was estimated that the CH 3 Cl emission from tropical plants could account for 30 -50% of the global CH 3 Cl emission (8). To accomplish an accurate estimation of CH 3 Cl production in the atmosphere through "bottom-up" approaches, elucidating the mechanisms and physiological functions of CH 3 Cl emission from plants will be important.The biological synthesis of methyl halides has been demonstrated mainly by biochemical analyses. The enzymatic activities that transfer a methyl group from S-adenosyl-L-methionine (SAM) 2 to halide ions (Cl Ϫ , Br Ϫ , I Ϫ ), which synthesize methyl halides, were first discovered in cell-free extracts of Phellinus pomaceus (a white rot fungus), Endocladia muricata (a marine red alga), and Mesembryanthemum crystallinum (ice plant, a halophytic plant) (9). Enzyme purification and cDNA cloning of the methyl chloride transferase (MCT) was first reported with Batis maritima, a halophytic plant that grows abundantly in salt marshes. As high concentrations of ions such as Cl Ϫ are often detrimental to plants, halophytic plants are considered to possess various salt tolerance mechanisms. MCT was hypothesized to control and regulate the internal concentration of Cl Ϫ , rich in the habitat in which halophytic plant grows (10, 11).In the meantime, purification of thiol methyltransferase (TMT), which methylates bisulfide (HS 2 The abbreviations used are: SAM, S-adenosyl-L-methionine; GC-ECD, gas chromatography equipped with an electron capture detector; GC-MS, gas chromatography/mass spectrometry; HOL, harmless to ozone layer; HPLC, high performance liquid chromatography; MCT, methyl chloride transferase; MS, Murashige-Skoog; RT, reverse transcription; SAH, S-adenosyl-Lhomocysteine; TMT, thiol methyltransferase.