What is a normalisation of a library?
A normalised library is one where every gene’s clone is present at the same abundance to each other. If you don’t do some form of normalisation or subtraction then for abundant RNAs you will have lots of cDNA clones, whilst rare RNAs, which are often the ones you are interested in, will have comparatively few cDNAs. When genes from a non-normalised or non-subtracted library are placed on an array you can have say 100-1000 spots representing the same abundant gene, whilst only 1 spot represents the rare potentially interesting gene.
