Chronic Post-Surgical Pain (CPSP) is a condition that involves experiencing pain and/or discomfort that lasts for more than 3 months following surgery without any explainable cause of this pain such chronic infection or pain caused from a condition preceding surgery. This condition negatively impacts health related quality of life and is associated with significant financial burden both for the patient and health care system. Patients undergoing common surgeries such as amputation, breast surgery, hernia repair, coronary artery bypass, and caesarean section can be prone to developing CPSP. CPSP may be caused by the release and expression of different cellular molecules called cytokines and chemokines that can cause a pain response when they are present in the body at certain levels. After surgical incision, tissue cells have been shown to release cytokines and chemokines, thus increasing the concentration of these molecules in the patient's blood. It is not entirely known what mechanisms cause the increased expression of chemokines and cytokines. One method that may play a role in in this expression is epigenetics which is the alteration of gene expression without permanently altering the structure of DNA. Unlike mutations, epigenetic changes are dynamic, possibly reversible, and are influenced by environmental and behavioral changes such as diet, exercise, disease, stress, toxin exposure etc. Epigenetic changes occur all throughout our life and have been associated with several disease processes such as cancer, diabetic complications, and chronic pain. At the molecular level, certain events take place that can regulate (increase or decrease) the expression of cytokines and chemokines, most notably DNA methylation of their promotor (which involves adding molecules to DNA that does not change its structure but changes it's activity). The investigators are conducting this study to determine if the alteration of specific cytokines are associated with the occurrence of CPSP and whether these cytokines are regulated by DNA methylation at their promoter. This study will take place at London Health Sciences Centre and will recruit up to 60 patients who will be undergoing a thoracotomy procedure (surgery of the chest area).