For getting insight into the mechanisms controlling DNA methylation and active DNA demethylation in CLL, DNA methylation ‘writers’, DNMT1/3A/3B; ‘readers’, MBD2/4; ‘editors’, TET1/2/3; and the ‘modulator’ SAT1, were tested by real time quantitative (RTq)-polymerase chain reaction (PCR) on purified peripheral blood naïve B cells, CD5 B1 cells and memory B cells from 6 healthy controls and B cells from 26 CLL patients for whom we have also determined the IGHV mutational status (Figure 3). The gene discussed is TET1; the disease is B-cell chronic lymphocytic leukemia.