  Torsion-rotation transitions in molecules exhibiting hindered internal
rotation possess enhanced sensitivities to a variation of the
proton-to-electron mass ratio. This enhancement occurs due to a cancellation of
energies associated with the torsional and rotational degrees of freedom of the
molecule. This effect occurs generally in every internal rotor molecule, but is
exceptionally large in methanol. In this paper we calculate the sensitivity
coefficients of methyl mercaptan, the thiol analogue of methanol. The obtained
sensitivity coefficients in this molecule range from $K_\mu=-14.8$ to $+12.2$
for transitions with a lower-level excitation energy below 10\,cm$^{-1}$.
