Invasive fungal infections in allogeneic hematopoietic stem cell transplant (allo-HSCT) recipients on antimicrobial prophylaxis is a major cause of infectious mortality, although its underlying pathophysiological mechanisms remain unclear. In a new report, Zhai and colleagues provide evidence that heteroresistance drives breakthrough Candida parapsilosis bloodstream infections in allo-HSCT recipients receiving micafungin prophylaxis.