Distribution of gamma- and beta-globin mRNA during postnatal development. The copy number (× 1000) of cDNA for individual reticulocytes sorted from (A) an umbilical cord, (B) a 2-month old infant, and (C) an adult human. Black bars represent gamma-globin gene–silenced (S) reticulocytes. Colored bars represent the nonsilenced (NS) reticulocytes with beta-globin copy number represented in red and gamma-globin copy number in yellow. The light gray line reflects a mean copy number for each donor. (D) Percentage of gamma-globin gene–silenced reticulocytes (•, solid line; gamma-globin mRNA below detection limit of 20 copies per cell) versus those reticulocytes containing low levels of gamma-globin mRNA (open boxes, dashed line; 21-100 copies of gamma-globin mRNA per cell) arranged according to donor age (x-axis). (E) Gamma-globin level among the nonsilenced populations expressed as a percentage of the total (gamma + beta; y-axis) arranged according to donor age (month; x-axis). The smooth trend lines were drawn using averages for each 10-week interval. In panels D and E, the postnatal (PN) age of the donors is shown on the x-axis with vertical gray lines at 6 months. A indicates adult.