A quantum mechanical study of all cis-syn cyclobutane pyrimidine photodimers including the normal and rare tautomeric forms of bases has been performed using the ab initio method at HF/6-31G(d,p), MP2(fc)//HF/6-31G(d,p) and MP2(fc)/6-31G(d,p) levels. A puckering angle of the cyclobutyl ring and twist angle of pyrimidine rings with respect to each other is well described by these calculations. It is predicted that in the gas phase all photodimers containing the rare imino form of cytosine are more stable than those containing its normal form. The Monte Carlo simulations show that the dimer containing the imino form of cytosine is more stabilized by water cluster than that containing its amino forms. The possible biological significance stems from the fact that the cytosine in the dimer directs the incorporation of adenine in the complementary strand during replicative bypass. Data obtained point to the cytosine tautomerism as a possible mechanism for the origin of UV-induced mutation.