UVA photoactivation of DNA containing halogenated thiopyrimidines induces cytotoxic DNA lesions.

•Growing cells incorporate thio-iodo-deoxyuridine and thio-bromo-deoxyuridine into DNA.•They are non-toxic but act as powerful UVA photosensitisers.•UVA lesions include DNA-protein and DNA–DNA crosslinks.•Singlet oxygen is involved in the formation of this potentially lethal damage.•Thio-halo-deoxynucleosides offer a potential selective therapeutic option.