CD19 and B-cell chronic lymphocytic leukemia: However, it is essential to keep in mind that also putative tumor-free material (i.e. saliva or CD19 negative blood cells) can be contaminated by CLL cells [103], active myeloid malignancy precursors (e.g., therapy-related myelodysplastic syndrome [104] or myeloproliferative neoplasm) or clonal hematopoiesis of indeterminate potential.