The hypothesis that will be tested in prospective studies with all available patients plus controls from two neonatal intensive care units is that the low responding FcgammaR genotypes (FcgammaRIIA, FcgammaRIIIA, FcgammaRIIIB) are more frequent in infants with late onset sepsis. The outcome will be to estimate the risk factor for late onset sepsis derived from expression of FcgammaR gentoype.