While cleavage of the Fas ligand is thought to modulate host immune responses by inhibiting apoptosis during the later proinflammatory stage of infection (44), the finding that Y. pestis lacking Pla exhibits significantly attenuated growth as early as 24 hpi led us to investigate the role of Pla during the earliest events that define pneumonic plague. The gene discussed is FASLG; the disease is pneumonic plague.