CD28 and cytomegalovirus infection: Results of CTLA-4 (318 C/T, 49 A/G, 1722 C/T, 1661 A/G), PD-1 (PD1.1 A/G, PD1.3 A/G, PD1.9 C/T), ICOS ( 693 A/G, +1720 C/T), CD28 (+17 C/T) single nucleotide polymorphism determined in 70 liver transplant subjects with or without CMV infection.