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In page Glutathione S-transferase:

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The early nomenclature for GSTs referred to them as “Y” proteins, referring to their separation in the “Y” fraction (as opposed to the “X and Z” fractions) using Sephadex G75 chromatography.[3] As GST sub-units were identified they were referred to as Ya, Yp, etc. with if necessary, a number identifying the monomer isoform (e.g. Yb1). Litwack et al proposed the term “Ligandin” to cover the proteins previously known as “Y” proteins.[4]