 As shown by Wittschieben et al., when DDB2 is in a complex with DDB1, forming the heterodimer DDB, this complex binds strongly to DNA containing one type of UV light-induced photoproduct [the (6-4) photoproduct], to DNA with an abasic site, to DNA containing mismatches without a covalent lesion, and to “compound” lesions containing both mismatches and lesions.The heterodimer DDB binds with intermediate strength to DNA containing another UV light-induced photoproduct (the cyclobutane pyrimidine dimer), and binds weakly to DNA that has no DNA damage.The DDB2 component of the heterodimer contains the specificity for binding to damaged DNA, since a heterodimer DDB complex containing amino acid substitutions in the DDB2 subunit, as found in XP-E patients, is very deficient in binding to damaged DNA.