average count to be added to each observation to avoid taking log of zero. Used only if log=TRUE.
vector of length nrow(x) giving gene length in bases, or the name of the column x$genes containing the gene lengths.
other arguments that are not currently used.
CPM or RPKM values are useful descriptive measures for the expression level of a gene.
By default, the normalized library sizes are used in the computation for DGEList objects but simple column sums for matrices.
If log-values are computed, then a small count, given by prior.count but scaled to be proportional to the library size, is added to x to avoid taking the log of zero.
The rpkm method for DGEList objects will try to find the gene lengths in a column of x$genes called Length or length.
Failing that, it will look for any column name containing "length" in any capitalization.