Because GM-CSF is secreted by Kras-expressing PDA cells, impacts macrophage Arg1 expression (Park et al., 2019), and activates PI3K (Hamilton, 2019), we postulated that cancer cell GM-CSF may be activating macrophage Arg1 expression through the PI3K–AKT pathway. The gene discussed is ARG1; the disease is Patent ductus arteriosus.