thanks Kevin for your answer,
It is exactly what I am looking for! In your package, you are using loadings to classify and calculate correlations between PCs and category names(eg. Size, Grade etc.). In my case, only 1 category : log2(fold-change). I do not know if log2 ratio is linear or maybe not, but I can use rank test anyway.
This will give us the information about which PC correlates with fold change and then identify important gene.
