In CLL patients, CD19+ cells were largely, and as expected, CD19+ CD5+ light chain restricted (either Ig k+ or Ig k-, Figure 1A, middle panel and Supplementary Figure 2C) whereas in age-matched HC the frequency of CD19+ CD5+ was less than 1.0% (0.83 ± 0.67% - range 0.24%-2.37%) of total PBMC (Figure 1A, middle panel and Supplementary Figure 3A) and polyclonal (Supplementary Figure 2B). Here, CD19 is linked to B-cell chronic lymphocytic leukemia.