Biomodels database.(A) The positive feedback loops with one species. Among the 32 loops with one species, the frequencies are 19 and 13. (B) The positive feedback loops with 2 species. Among the 108 loops with 2 species, the frequencies are (from top left, row by row): 35, 16, 9, 16, 13, 23. (C) The histogram shows the size (number of species) distribution among the 341 positive feedback loops found in the 64 models.