Table 4. Candidate reference genes for normalization listed according to their expression stability calculated by the NormFinder program.

Ranking order	Gene name	Stability value
1	UBC	0.365
2	PPIA	0.370
3	HPRT1	0.436
4	H3F3A	0.473
5	GAPDH	0.480
6	TBP	0.566
7	GUSB	0.586
8	POLR2A	0.653
9	ACTB	0.657
10	B2M	0.682

High expression stability is indicated by a low stability value as an estimation of both the intra- and the intergroup variation of individual gene, according to the model-based approach developed by Andersen et al. [17].