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Enter your search terms separated with commas. A search term may have more than one word. So "sigma factor, regulator" are just two terms NOT three.
Most of the time you probably want to AND your search terms. In general ANDing should give fewer results than ORing. To get a feel, try "sigma factor, regulator" first with AND and then with OR. Note the difference in the number of results you get.
Also, "protease inhibitor" and "protease, inhibitor" will give different results. In the first case there is only one term and an exact phrase match is being carried out, while in the second there are two terms being searched.