Why do I have to add beta-mercaptoethanol (beta-ME) to lysis Buffer RLT of the RNeasy Kits?
FAQ ID -101

When working with RNA, care must be taken to avoid degradation by RNases, which are extremely stable and active. Intracellular RNases are released during the lysis step of the RNA isolation procedure and must be rapidly and thoroughly inactivated to obtain high-quality RNA.

Beta-mercaptoethanol (ß-ME) is a reducing agent that will irreversibly denature RNases by reducing disulfide bonds and destroying the native conformation required for enzyme functionality. In combination with the strong, but temporary denaturing effects of guanidinium isothiocyanate (GITC) contained in buffer RLT of the RNeasy Kits, any RNases present in the material to be extracted from will be completely inactivated.