Competitive factor (or killer factor) refers to the natural ability of some yeast strains to produce proteins (killer factor) that inhibit the growth of other strains. Generally, yeast strains are categorized as one of the following: 1) Active – produces killer factor and is not sensitive to killer factor; 2) Neutral – does not produce killer factor and is not sensitive to killer factor; 3) Sensitive – does not produce killer factor and is inhibited by killer factor.