UltraPure™ Guanidine Isothiocyanate Solution consists of 4 M guanidine isothiocyanate, 50 mM Tris-HCl (pH 7.5), 25 mM EDTA. It may be used as part of a protocol, based on the guanidine isothiocyanate procedure first described by Chirgwin et al. (1) and later modified by Han et al. (2), for isolating intact mRNA from tissues or cultured cells.