I have downloaded RNA-Seq expression z.scores from TCGA datasets on cBP. For some genes, they have a z.score value of NaN. Does that mean that the expression level of that gene was 0? Or does it mean something else? I couldn't seem to find the answer online, thanks for your help!
by default z-score is centering and then dividing with the standard deviation. My guess would be that standard deviation was 0. It occurs when the expession level is 0, but may also occur in other situations, however, everything except 0 looks unrealistic.