MC_CORES and mc.cores for parallel package

Hi,

 From the documentation of ?options

Options set in package parallel
These will be set when package parallel (or its namespace) is loaded if 
not already set.

mc.cores:
a integer giving the maximum allowed number of additional R processes 
allowed to be run in parallel to the current R process. Defaults to the 
setting of the environment variable MC_CORES if set. Most applications 
which use this assume a limit of 2 if it is unset.

Then I try:
 > getOption("mc.cores")
NULL

I suspect that my environment variable MC_CORES is not set. I test and 
that's right:
 > Sys.getenv("MC_CORES")
[1] ""

Then I do:
 > Sys.setenv(MC_CORES=4)
 > Sys.getenv("MC_CORES")
[1] "4"

But when I try again getOption("mc.cores"); it does not change:
 > getOption("mc.cores")
NULL

Probably I do something wrong but I don't see what !

Thanks

Marc