| The number of proteins participating in a pathway is known through genetic complementation analysis. |
| "Auxotrophs" are mutations that cause the need for supplment of certain nutrient (such as amino acids) in order for an organism to survive and to grow. |
| The number of different "auxotrophs" that can be generated in a given nutrient suggests the number of proteins (or genes) that are involved in the biosynthesis of that particular nutrient. |