Average cycle threshold (Cq) values of candidate reference genes tested in PBMCs under different conditions. The values are the average RT-qPCR cycle threshold numbers (Cq values). The box plot indicates sample’s range, median, normality of the distribution, and skew of the distribution. Letters indicate a significant difference in average Cq value. Average Cq values that have the same letter are not significantly different (P > 0.05).