CPA needs bioactivation in the liver to 4OH-CPA by CYP enzymes, particularly CYP2B6, which occurs mainly with *1/*1 alleles (wild type). As a result of SNPs, variant alleles occur, including the *6 variant associated with reduced levels of the CYP2B6 enzyme. In CLL patients treated with FC within a controlled clinical trial, Johnson et al¹  found that patients with at least one *6 allele had lower complete response rates, less toxicity, and a trend of shorter PFS than *1/*1 subjects, indicating lower CPA bioactivation by the CYP2B6 *6 enzyme variant. In contrast, no differences in outcome were detected in patients treated with chlorambucil or fludarabine. Professional illustration by Paulette Dennis.