[BioC] when to impute the missing value?

[De-Jian,ZHAO]

Dear all,

Microarray experiments easily have missing values. The R packages
impute and pcaMethods both can impute the missing values. I wonder
at which step we carry out the imputation.

After reading data into R?
Before normalization?
Ater finding the differentially expressed genes?
When required to do so,e.g. PCA and SVD?
...

Thank you for any advice.

Zhao