It should be noted that in some cases it may not be possible to find an ri,j∈R that is differentially phosphorylated in comparison to xt. This may happen because of a small and incomprehensive repository or because of an attempt to (de)phosphorylate a peptide that is highly (de)phosphorylated and further (de)phosphorylation is not possible. In this case, Algorithm 2 reports the peptide as not differentially phosphorylated and the value for yt′ remains unchanged.