How DegSeq calculated log2(Fold change) when one of the read counts is zero...

[mhjung]

Dear all,

I'm new to the forum and new to the statistical analysis of ESTs data.
I was reading a paper about differential gene expression and I took a look at one of their additional file data. They use DegSeq to analyse their data.
I was wondering if someone could explain me how log2 of the fold change is calculated when one of the samples gives a read count of zero...
Here are some data they present:

_____________________log2___________log2_____________________________
#ReadsA____ReadsB___(Fold_change)___(Fold_change)___z-score____p-value
____________________________________normalized_______________________
174__________0___________8,44___________10,86_______19,04_____8,39E-81
197__________1___________7,62___________10,04_______21,59_____2,34E-103
63___________0___________6,98___________9,39________12,77_____2,29E-37
38___________0___________6,25___________8,66________10,38_____2,95E-25
138__________2___________6,11___________8,52________19,95_____1,64E-88

Can someone give me a help to understand this? (Sorry if this is basic and I miss it...)

Cheers,
MHJung