NCAM1 and viral infectious disease: The expansion of the CD56+/CD57+ subset might be related to the numerous episodes of viral infections that have been observed in Pt1, suggesting that recurring exposures of HPS2 patients to viral pathogens might have led to depletion of the CD56bright/CD16−/CCR7+ subset and, secondarily, to expansion of the of CD56+/CD57+ memory NK subset [28].