Cytomegalovirus (CMV) serostatus is measured using solid-phase enzyme immunoassay at Stanley Laboratory of Developmental Neurovirology at the Johns Hopkins School of Medicine. Solid-phase enzyme immunoassay is used to measure the level of immunoglobulin G class antibodies to CMV sera. Optical density of the ensuing enzyme-substrate reaction is quantified by means of a microplate colorimeter at a wavelength of 450 nm. Researchers performing the assays are blinded to participants’ clinical characteristics. Specimens are classified as seronegative if the immune status ratio, determined by comparison with defined standards, is less than 1.0, or as seropositive if the ratio is ≥1.0.