RAD18 binds to FANCD2 in vivo. (A) Immunoprecipitation of endogenous FANCD2 from H1299 whole-cell extract followed by immunoblot for endogenous RAD18 shows that RAD18 binds to FANCD2. This interaction is present in untreated cell extracts; however, treatment with MMC slightly increases RAD18 and FANCD2 interaction. (B) HA-tagged RAD18 was expressed in H1299 cells using adenoviral vectors. Immunoprecipitation of endogenous FANCD2 followed by immunoblot for HA showed that exogenously expressed RAD18 interacts with FANCD2. (C) Immunoprecipitation of endogenous FANCD2 from FANCA-deficient and corrected whole-cell extracts followed by immunoblot for endogenous RAD18 showed that RAD18 and FANCD2 interact in both FA-A cells and corrected cells. This interaction is therefore core complex–independent, and RAD18 likely interacts with nonubiquitylated FANCD2.