In a study of 398 female breast cancer patients, Doong et al (2014) also reported an association between patients homozygous for the high-producing G allele of IL-6 (rs2069845) and a symptom cluster of pain, fatigue, sleep disturbance, and depression, potentially occurring as an interaction between SNPs for IL-13 and tumor necrosis factor-alpha (TNF-α). Here, TNF is linked to major depressive disorder.