The mu class of GSTs in humans are the products of three gene loci (GSTM1, GSTM2, and GSTM3), and share a very high level of amino acid sequence homology. The GSTM1 locus, in turn, has three allelic forms. This recombinant M1-1 form is produced in an E. coli expression system and, following purification, retains high catalytic activity.

References:
Comstock, Johnson, Rifenbery, Henner, J. Biol. Chem. 268:16958-16965, 1993.