Assessment of OPN mRNA levels in FACS-sorted macrophage populations. Graph demonstrates data derived from quantitative RT-PCR of OPN mRNA from sorted macrophage subtypes. Macrophage populations were defined and sorted (according to F4/80 and Ly6C levels) from a single sample of pooled muscles collected from six 8-wk-old mice and characterized as shown in Fig. 5. Expression of OPN was normalized to GAPDH. Samples were run in triplicate to measure the expression of both genes.