DNA Damaged binding protein 1 (DDB1) is a highly conserved protein of around 125 kDa. It serves as a substrate adaptor subunit to a CUL4-based E3 ubiquitin ligase within the ubiquitin proteasome pathway. However, based on a set of three beta-propellers, the protein is able to mediate various protein-protein interactions, suggesting that it participates in many developmental and physiological processes in the plant. Arabidopsis encodes for two closely related DDB1 proteins, named DDB1a and DDB1b. While loss-of DDB1a does not severely affect development, loss-of DDB1b has been reported to result in an embryo lethal phenotype. Here we describe two novel ddb1b T-DNA insertion mutants that are not embryo lethal, which we utilized as genetic tools to dissect DDB1b from DDB1a function. Information generated by these studies showed that the C-terminal part of the DDB1 proteins is critical for specific protein-protein interactions. In addition, we demonstrated that DDB1a, like DDB1b, is critical for embryo development, and that both proteins have distinct functions in whole plant development.