Matrix of gene expression data, where columns correspond to samples
and rows to genes. The data should be properly normalized beforehand
(and log- or otherwise transformed). Missing values are not allowed.
Gene and sample names can be included as the row and column
names of xx.
formula
Model formula for the linear model.
model.dat
Data frame that contains all the variable information for each sample.
test.genes
Vector of gene names or gene indices specifying a gene set.