The sample size was calculated using EPi Info 2000, version 1.1.2 (Center for disease control and prevention, Atlanta, Georgia, USA) based on proportion of telomere shortening in normal subjects range from 0-1% and it is expected to be 14 % in patients with coronary artery disease (Daily News, American Heart Association, 2016) using 90% confidence interval and 80% power, the sample size is 54 cases of coronary artery disease and 54 control subjects (total 108).