Natural selection of clones. Mutations involving granulocyte colony-stimulating factor receptor (GCSFR) emerge during the course of severe congenital neutropenia (SCN) treated by recombinant human G-CSF. However, some mutations thrive and persist, while others die off. Professional illustration by Debra T. Dartez adapted from Figure 4 in Beekman et al, which begins on page 5071.