Mean insertion and deletion frequencies in essential and non-essential proteins plotted against minimum indel length. Mean insertion and deletion frequencies were calculated for essential and non-essential query proteins aligned to proteins from the 22 bacteria or 15 eukaryote species. The t-test statistic is shown for the minimum indel lengths that were found significantly more often in essential (blue bars) than non-essential (purple bars) proteins. Significance was set at P < 0.05. Note that no such difference was observed in insertions within B. subtilis proteins.