ICOS and cytomegalovirus infection: Considering the great effect of co-stimulatory molecules on T-cell function and assuming that alteration of gene polymorphisms expression or function would possibly influence the immune response, this study was conducted to investigate the probable connection between 10 polymorphisms of co-stimulatory molecules (four polymorphisms in CTLA-4, three polymorphisms in PD-1, two polymorphisms in ICOS, and one polymorphism in CD28) with HBV and CMV infection in liver transplant patients.