[BioC] Getting the list of NA values from an MAList in limma

[michael watson (IAH-C)]

Sorry, this is due to my lack of knowledge of R I suspect!

I have an MAList object in limma containing several arrays.  I access
the column of M values thus:

MAn[,3]$M

Where Man is my MAList object.  What I want to do is create a factor
which tells me which entries of that M column are NA and which aren't -
I basically want to highlight on a background plot those spots which are
going to produce a negative value after background subtraction, and
therefore an NA value in the MAList object.

Thanks in advance for the help

Mick