The approval of Zokinvy in Japan is a significant regulatory milestone for Eiger BioPharmaceuticals, directly leading to a $500K payment. This news is likely to be viewed positively by investors as it not only represents immediate revenue but also opens up a new market for Zokinvy, potentially leading to future sales and revenue growth. The confidence in the analysis is high due to the direct financial impact and the clear connection between the regulatory approval and the milestone payment.