FIGURE 1: A symbolic illustration of different levels of essentiality of the Saccharomyces cerevisiae genes.

(1) Conditional essential: when all other roads are closed, this alternative road becomes essential.

(2) Essential: the only available road to your destination.

(3a) Redundant essential: two duplicated/similar roads to your destination; (3b) Redundant essential: two different roads (pathways), but to the same destination; (3c) Redundant essential (synthetic sick): Two similar/different roads, plus a less preferred road, when the main roads become unavailable. The last part of the road (marked as red) represents the absolute essential.