Expression levels of p16^INK4A as a function of age in healthy and leukemic CD34⁺ cells. The expression levels of p16^INK4A mRNA of CD34⁺ cells of adult healthy controls and CD34⁺ cells of adult AML samples is shown. × indicates healthy adult sorted CD34⁺ cells; and ○/●, sorted CD34⁺ AML cells. Spearman rank correlations were calculated between age at diagnosis and p16^INK4A mRNA expression levels in AML samples (P = .025, ρ = −0.308, n = 53). The dotted line represents the regression line for the adult healthy sorted CD34⁺ cells (n = 16); and solid line, the regression line for the sorted CD34⁺ AML cells. We investigated whether the expression level of p16^INK4A mRNA in the sorted CD34⁺ cells of 53 AML samples was above or below the expected p16^INK4A mRNA expression in the sorted CD34⁺ cells of the adult healthy controls (n = 16). ● represents a p16^INK4A expression lower than the age-matched p16^INK4A expression in healthy controls; and ○ represents a p16^INK4A expression higher than the age-matched p16^INK4A expression in healthy controls. The majority of patients with low p16^INK4A expression (●) were older than 55 years.