Isoschizomers are a type of restriction enzyme. Over 50% of commercially available restriction enzymes have some degree of cleavage sensitivity when their target recognition sequence overlaps a DNA methylation site, whether fully or partially. The restriction enzyme performance of isoschizomers can vary significantly to methylated DNA in the target region, from zero effect (all methylated DNA sequences get digested) to partial inhibition (only some methylated DNA sequences are digested) or complete blocking (no methylated DNA sequences are digested). This variation in sensitivity to methylation is the reason why you must take DNA methylation sensitivity into consideration when evaluating isoschizomers for your experiment.