We have recently shown that analysis of BAT-26, was sufficient to establish the Replication Error status of colorectal tumors and cell lines without the need for matching normal DNA. BAT-26, a poly(A) tract in the 5th intron of the hMSH2 gene, does not present significant size variation either between the alleles of one individual or between alleles of different individuals. In colorectal tumors without defects in the replication error system (RER- phenotype), BAT-26 is also quasi-monomorphic. On the contrary, in RER+ colorectal tumors, BAT-26 shows unstable shortened alleles. In order to see whether this behaviour was specific for BAT-26, or was a more general phenomenon for mononucleotide repeat microsatellites, we analysed eight other mononucleotide repeats. In control individuals (72 samples) and in RER- colorectal tumors and cell lines (55 samples), these microsatellites were polymorphic, dimorphic, quasi-monomorphic or monomorphic, indicating that the quasi-monomorphic nature of BAT-26 was not a general rule. All of them showed a tendency to be shorter in RER+ colorectal tumors and cell lines (19 samples), but only quasi-monomorphic and monomorphic mononucleotide repeats could be used to determine the RER status of tumors without matching normal DNA, although with a lower efficiency than BAT-26 due to either a smaller range of shortening in RER+ tumors or to a larger number of false negative cases.