As shown in Fig. 3A, expression of Nluc activity during YFVΔSK/Nluc infection was blocked by Pyr-41, a potent, specific, and irreversible inhibitor of the ubiquitin-activating enzyme E1 (36), confirming that ubiquitylation is required for an early, prereplication event in the YFV life cycle. Here, UBA1 is linked to infection.