In the prosecution of a generic driving under the influence case (Veh. Code, sec. 23152, subd. (a)), introduction of competent evidence showing that the 2,100 to 1 conversion ratio may have yielded an inaccurate representation of defendant’s blood-alcohol level is permissible. Vehicle Code section 23152, subdivision (a) prohibits driving under the influence of alcohol. For the purpose of subdivision (a) (generic driving under the influence), a presumption of intoxication arises if testing shows the presence of .08% or more of alcohol in the driver’s blood. A breath sample must be converted to derive a blood-alcohol level. The conversion factor, known as a “partition ratio” reflects the relationship between alcohol measured in a persons’s breath and alcohol in the blood. Breath testing machines in California incorporate a partition ratio of 2,100 to 1, which means the amount of alcohol in 2,100 milliliters of breath is considered equivalent to the amount of alcohol in 1 milliliter of blood. But the partition ratio varies widely in the general population and can also vary in the defendant himself. If defendant’s own partition ratio at the time of testing is lower than this standard ratio, conversion of the breath result using the statutory formula produces an artificially high measure of his blood alcohol. Further, evidence about the variability of partition ratios in the general population is relevant to raise a reasonable doubt about the accuracy of a defendant’s converted blood-alcohol level. Because the language dealing with partition ratio evidence, as contained in subdivision (a), creates a rebuttable presumption (as opposed to the language in subdivision (b), per se driving under the influence, where the language defines the crime, itself; i.e., driving with more than .08%), defendant is entitled to challenge the presumption by raising a reasonable doubt as to whether the test result was an accurate measure of his blood-alcohol level.