A 72-year-old woman has an elevated WBC count on routine testing. The manual differential of the WBC notes an increased number of mature lymphocytes. She reports no recent illnesses or infectious symptoms, and her past medical history is significant for hypertension, osteoarthritis, chronic stable angina, and dyslipidemia. She is taking appropriate medications for these conditions and has no change in symptoms or medications. Her physical examination is normal. Which of the following features best characterizes CLL?