Id1 counteracts E2-2–mediated luciferase activity. (A) Id1ΔHLH does not perturb E2-2–induced MCKpfos-luc activity. MEECs were transfected with MCKpfos-luc, E2-2, and either Id1 or different amounts of Id1ΔHLH. (B) Id1 relieves the inhibition of pGL2b-VEGFR2-luc (−166 bp/267 bp) activity by E2-2. MEECs were transfected with pGL2b-VEGFR2-luc (−166 bp/267 bp), Id1, and E2-2. (C) Id1 disrupts E2-2 homodimer formation. The experiment was performed as described in Figure 1A. E2-2 homodimer formation (top panel) and E2-2/Id1 heterodimer formation (second panel) are shown. Expressions of Myc-E2-2 (third panel) and Myc-Id1 (bottom panel) were evaluated using an anti-Myc 9E10 antibody, and the expression of Flag-E2-2 was shown using an anti-Flag M5 antibody (fourth panel). (D) Id1 disturbs the preexisting E2-2 homodimer formation. Left panel: Illustration of how cell lysates were prepared from each dish in which indicated plasmids were transfected in COS7 cells. Right panel: After each cell lysate was mixed, the experiment was performed as described in Figure 1A. E2-2 homodimer formation (top panel) and E2-2/Id1 heterodimer formation (second panel) are shown. The expression of Flag-E2-2 was shown using an anti-Flag M5 antibody (third panel). Expressions of Myc-E2-2 (fourth panel) and Myc-Id1 (bottom panel) were evaluated using an anti-Myc 9E10 antibody.