Abstract: Gene deletion studies in yeast have shown that only approximately 18% of its genes are essential for survival under standard laboratory conditions. This unexpectedly high fraction of genes with apparently no deletion effect has many practical and fundamental implications, and it is subject of considerable interest. Here, we briefly review some of the complementary models proposed to explain the robustness observed in biological networks. We also present and analyse a collection of well-documented cases of gene pairs with capacity to compensate the deleterious effects caused by the inactivation of one of the partner genes, and suggest the molecular bases of how these functional compensations might occur at the protein level.