Immunofluorescence analysis of mouse tissuesTo analyze vascular morphogenesis in the neonatal retina, eyes were dissected and fixed in 4% paraformaldehyde (Sigma) for 1 h on ice, washed in PBS before dissection of retinas. After blocking in TNB buffer (0.5% blocking reagent (PerkinElmer, FP1012), 150 mM NaCl, 100 mM Tris-HCl (pH 7.4), 0.4% Triton X-100) for 2 h at room temperature, the retinas were incubated with Alexa-Fluor-conjugated isolectin B4 (Invitrogen, I21411, 20 μg ml−1 in TNB) and primary antibody (if required) overnight at 4 °C. Anti-collagen IV (AbD Serotec, 2150–1470, 1:400), anti-ERG (Abcam, Ab92513, 1:200) or FOXO1 (Cell Signaling Technology, 2880, 1:100) and then Alexa-Fluor 546-conjugated secondary antibody (Invitrogen, 1:400) were applied in sequence. Afterwards, retinas were flat-mounted with Fluromount-G (Invitrogen) and examined by Olympus FV1000 or Zeiss LSM 710 confocal microscope. To label the proliferative endothelial cells with EdU, pups were injected i.p. with 60 μl EdU (Invitrogen, C10338, 0.5 mg ml−1 in PBS) 3 h before experiments. EdU detection was performed by Click-iT EdU Cell Proliferation Kit for Imaging (Invitrogen, C10338). Retinas were stained by In Situ Cell Death Detection Kit TMR red (Roche) as the manufacturer’s instruction for determination of apoptotic ECs.In implanted tumor assays, mouse tissues were fixed in 4% paraformaldehyde overnight at 4 °C, washed in PBS, equilibrated in 30% sucrose overnight, embedded in OCT (Sakura), and sectioned at 5 μm. Slides were treated with 0.2% Triton X-100 in PBS and incubated in blocking solution (1% BSA, 5% goat serum in PBS) for 1 h at room temperature. Primary antibodies anti-CD31 (BD Biosciences, 553370, 1:200) and anti-collagen IV (AbD Serotec, 2150–1470, 1:200) were incubated overnight at 4 °C, followed with Alexa-Fluor 488- and Alexa-Fluor 546-conjugated secondary antibody (Invitrogen, 1:400) stainings for 2 h at room temperature. Samples were stained by DAPI solution for nuclei and then mounted by Fluromount-G. Slides were examined by Olympus BX-53 upright or Zeiss LSM 710 confocal microscope. Image analysis was accomplished with ImageJ. To assess tumor vessel density, CD31- positive area was measured and normalized with the tumor area. For analysis of vessel regression, the collagen IV-positive area was measured and divided by the CD31-positive area as previously reported24.