I tried to filter out those genes which do not express by gene filter method as follows:
f1 <- pOverA(0.25, 3.5)
ffun1 <- filterfun(f1)
flrGene <- genefilter(data,ffun1)
Then it gives me zero , why ? means I should keep all the genes ? is there any other method to remove those genes with very low expression over samples ?