Since the discovery of genes, we have learned that there are portions of chromosomes that don't represent the code for anything functional (i.e. a protein). Telomeres are segments of non-coding, repeating units of DNA found at the ends of our chromosomes, but they serve a variety of different purposes, including making sure our chromosomes align properly during replication. Also, since replication of DNA inevitably introduces some degree of error to the sequence, one of the causes of gene mutation, and since these errors occur more frequently on the ends, the telomeres protect important gene sequences from becoming mutated. Telomeres shorten gradually as our cells replicate and make copy after copy of DNA. Some evidence suggests telomere shortening might be an early sign of cancer, as is indicates frequent cell reproduction. Other research shows that once the telomeres become too short, the enzyme telomerase can lengthen them again, allowing cell replication to continue. Normal cells don't have telomerase activity and control of the enzyme might be an answer to tumor suppression in cancer treatment. Telomeres also play a role in epigenetics, as they appear to be involved in the protection of histone structures, and their lengths are correlated to certain epigenetic markers.