DTT is frequently used to reduce the disulfide bonds of proteins and, more generally, to prevent intramolecular and intermolecular disulfide bonds from forming between cysteine residues of proteins. DTT is oftentimes used along with sodium dodecylsulfate in SDS-PAGE to further denature proteins by reducing their disulfide bonds to allow for better separation of proteins during electrophoresis.
DTT is used as a reducing or "deprotecting" agent for thiolated DNA. The terminal sulfur atoms of thiolated DNA have a tendency to form dimers in solution, especially in the presence of oxygen.