In contrast, the catalytic activity of the CYP2A6-specific coumarin hydroxylation varied in four measured breast cancer microsomes between 1.6 and 4 pmol min−1 mgprotein−1, whereas in human liver microsomes this activity was approximately 24 nmol min−1 mgprotein−1 (Hellmold et al, 1998). Here, CYP2A6 is linked to breast cancer.