(A) The essential and nonessential excess retention as a function of logarithmically binned connectivities of proteins shows statistically significant and monotonic trends (ER_k ∼ γlog k, essential: γ = 1.386, nonessential: γ = –0.371). (B) The trend of excess retention of essential proteins is further enhanced if we focus on proteins that simultaneously have orthologs in higher eukaryotes ([TeX:] \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\bar{{\gamma}}}=2.128\) \end{document} ). (C) However, proteins that have orthologs in the reference eukaryotes and are dispensable for the cell's survival fail to show any significant excess retention signal.