FIGURE 1: M-factor function is stringent, whereas P-factor function is more relaxed, extending beyond species. M-factor and P-factor, secreted by Minus and Plus cells, respectively, are recognized by their corresponding receptor on the surface of cells of the opposite mating type. Whereas the specificity of M-factor recognition is extremely stringent, that of P-factor recognition is more relaxed, allowing cross-reactions to occur even between S. pombe and S. octosporus. P-factor is interchangeable between the two species, but not M-factor. Sp, S. pombe; So, S. octosporus.