The aim of this proposal is to identify immune biomarkers, genetic risk, and the clinical consequences of low count monoclonal B-cell lymphocytosis (LC MBL), a common premalignant condition affecting up to 17% of European adults age\>40. LC MBL is a precursor to chronic lymphocytic leukemia (CLL), characterized by a circulating population of clonal B-cells. It is relatively understudied, despite emerging evidence of clinical consequences such as increased risk for life-threatening infections and lymphoid malignancies. Studies reported that male sex, age, family history of CLL, and CLL-susceptibility genetic loci were associated with LC MBL risk. These findings were reported in European ancestry individuals and have not been generalized to other thnicities. This study will provide this missing knowledge using a unique multi-ethnic Israeli population of Jews and Arabs that have one of the highest and lowest age-standardized incidence rates of CLL in the world, respectively, and characterized with different genetic backgrounds.