APLN and neoplasm: Apelin has previously been shown to stimulate tumor angiogenesis and is upregulated in tumor‐associated endothelial cells (Seaman et al, 2007; Wang et al, 2007; Liu et al, 2015), which we could confirm in endothelial cells isolated from MMTV‐NeuT tumors compared to normal mammary gland (Fig EV2A).