[BioC] How to Change labels to a print-tip boxplot ?

[Giulio Di Giovanni]

Hi guys, thanks to Jenny Drnevich I discovered how to create a boxplot of M 
values by print-tip group using limma package only:

boxplot(MAset$M[,j]~MAset$genes$Block)

Now my problem is : how to replace the xlabels (I mean, not the X-axis 
title) with my labels ? The default is obviously the Block number, and I 
would like to replace it with my vector of labels (grlab, character), the 
same length of the number of the boxes.

Yes I know that is more an R question, and even a stupid one, but I tried in 
several ways and I almost became crazy on that !!!
I tried with par() settings, axis(), adding "lab=", labels, tried to rename 
the xdata (names, rownames, etc).
If someone could help me, it will be great !!!

Thanks in advance,

Giulio