In this issue of Blood, Dahlin et al report on a minor circulating human mast cell (MC) progenitor cell population (lineage-negative [Lin−]/CD34hi/CD117int/hi/high-affinity immunoglobulin E receptor-positive [FcεRI+]), with an immature MC-like appearance, which is present in the peripheral blood (PB) of healthy individuals and of asthma subjects well controlled by treatment or with reduced lung function.