Using four different mathematical algorithms (geNorm, NormFinder, comparative ΔCq and BestKeeper) PPIB, TBP and RPL22 were identified as the most stable, reliable and suitable reference genes for normalization in relative RT-qPCR gene expression studies on human periodontal ligament fibroblasts, particularly in studies on orthodontic tooth movement (PPIB/RPL22) and periodontitis (Aggregatibacter actinomycetemcomitans, PPIB/TBP). The gene discussed is PPIB; the disease is periodontitis.