CNP and NTproCNP levels vary by age. Plasma levels of CNP and NTproCNP were determined by RIA for healthy children between the age of 2 months and 19.8 years of age. Open circles, individual data points; heavy line, median; light lines, 5th and 95th percentiles. Top left panel, NTproCNP levels for boys (n = 209), top right panel, CNP levels for boys (n = 178). Bottom left panel, NTproCNP levels for girls (n = 143), bottom right panel, CNP levels for girls (n = 140).