The other fact is the influence of cigarette smoking. Non-smoking patients have lower PPI compared to smokers, and heavy smokers have the highest PPI.[24][25] This finding runs in accord with the high rates of smoking among schizophrenic patients, estimated at 70%,[26] with many patients smoking more than 30 cigarettes a day.[27] Thus, smoking may be a way of self-medication. Some studies show association of schizophrenia with the CHRNA7 and CHRFAM7A genes, which code for alpha7 subunit of nicotinic receptors, but other studies are negative.[28][29] Contrary to the predictions, nicotine receptor alpha7 subunit knockout mice do not show disruptions in PPI.[30]