It has previously been found that, in yeast, gene essentiality is positively correlated with protein connectivity (number of interaction partners) but negatively correlated with the existence of gene duplicates and that highly connected proteins tend to have a low gene duplicability. Using data from human and mouse, we show here that, in mammals, the first of these relationships holds true, but unlike the second relationship in yeast, highly connected mammalian proteins tend to have a high gene duplicability, and there is no correlation between gene essentiality and gene duplication in mammals.