Telomeres are DNA repeats at the ends of chromosomes. Their function is to maintain the integrity of chromosomes. Telomeres are shortened every time DNA is copied. Once telomeres are exhausted, chromosomes are prone to mutation, leading to arteriosclerosis or cancer. 
Telomere DNA analysis of blood samples is a commonly used analysis method in the laboratory. It requires a large amount of blood, takes time and effort, and the accuracy of measurement is greatly affected by the technical level
 some people speculate that the length of telomere DNA can be used to calculate people's life span.