Background
Cullin1 (CUL1) belongs to the cullin family proteins that are scaffold proteins for the Ring finger type E3 ligases. Human has seven cullin proteins: CUL1-3, CUL4A, CUL4B, CUL5, and CUL7. Cullin1 is the mammalian homolog of Cdc53 from yeast. CUL1 has been shown to mediate ubiquitin dependent degradation of p21 Waf1/Cip1, cyclin D and IκBα. Each cullin protein can form an E3 ligase similar to the prototype Ring-type E3 ligase Skp1-CUL1-F-box complex. The Cullin-RING-finger type E3 ligases are important regulators in early embryonic development, as highlighted by genetic studies demonstrating that knock-out of CUL1, CUL3, or CUL4A in mice results in early embryonic lethality.