[Genome] Affymetrix All Exon Chips

[Pelleymounter, Linda L.]

I was looking at the Affymetrix All Exon Chips (tissue).  I noticed that
the scale is from -4.0 to 4.0, where a green color indicates
underexpression and a red color indicates overexpression.  What does the
black mean?  Does the black mean no hybridization?  or, does the black
mean expressed, but only at the median level?
I was specifically looking at the expression level within tissue of the
areas represented by the Affymetrix Exon 1.0 Probe Sets, 2411275 and
2411276.  

Thank you.
Linda