The highest reported prevalence of CLL-like MBL occurs in first-degree relatives of CLL patients. All epidemiological studies that have investigated the prevalence of CLL and other lymphoproliferative disorders in relatives of CLL patients have reported elevated risks of CLL in relatives with the largest study showing a 8-fold increase in risk.(10, 11) The first reports of MBL came from studies of apparently unaffected family members in the early 1990s.(12) More recently studies in both the UK(13) and U.S.(12) demonstrated a very high prevalence of CLL-like MBL in individuals with a family history of CLL who had normal blood counts (prevalence=13.5–18%). The UK group had used the same methodology for their studies of both CLL families and the general population and found a 4-fold increase prevalence of MBL in families with a genetic predisposition to CLL.(4, 13) For young adults aged 16–40 years the relative risk is 17-fold.(13) Although these studies(12, 13) clearly demonstrate a higher risk of MBL among unaffected individuals from CLL families relative to the general population, other recent studies suggest that first-degree relatives of individuals with sporadic CLL may also be at increased risk of MBL if they are over age 60. (14)
