Pyrimidine (6-4) pyrimidone photoproducts are one of major classes of DNA damage that occur upon UV exposure.  These photolesions have the ability to trigger numerous biological responses, some of them being of utmost importance for the cell viability.  The use of nucleobase thio analogues has been of tremendous importance to allow a precise formation mechanism determination of pyrimidine (6-4) pyrimidone photoproducts as well as to propose a reasonable mechanistic pathway for their repair by the (6-4) photolyases.  This review summarizes the results obtained during this last decade in the (6-4) adduct field using thionucleobases.