The main symptom of ankylosing spondylitis is pain, characterized by pain in the lower back and lumbar region. The pain occurs during rest and diminishes with activity; it is especially severe at night, often waking the sufferer, who then finds mild activity alleviates the pain enough to return to sleep. As the disease progresses, chest pain and restricted rib cage movement begin, due to the disease spreading to the thoracic vertebrae and eventually reaching the cervical vertebrae in later stages, resulting in difficulty moving the neck. Early physical signs include tenderness in the sacroiliac joints and paraspinal muscles. X-rays can reveal that the sacroiliac joints are often the first to be affected, usually showing sacroiliitis. Additionally, there is a particularly high positivity rate for HLA-B27, the human leukocyte antigen B27, which can reach up to 90%, whereas it is typically around 4%-9% in the general population.