The current pilot randomised controlled trial was designed to explore the impact of high-intensity interval training (HIIT) on cardiovascular fitness and markers of cardiac regulation (HRV), sympathetic nervous system activity (salivary (s) α-amylase (s-AA)), HPA axis (salivary cortisol (s-cortisol)), and mucosal immunity (salivary immunoglobulin A (s-IgA)) in breast cancer survivors. The gene discussed is CD79A; the disease is breast cancer.