<p> Estimating the "minimal gene set" for a cell to be viable is an important issue in understanding "living" cells, creating "artificial" cells, and revealing "ancient" cells. The minimal gene set is critical information for understanding a cell system and designing an artificial genome, which is an essential element for creating an artificial cell. Artificial cells can provide many clues to understanding primordial life on Earth. To reveal minimal gene sets, "essential genes" in many bacteria, which could not be removed from bacterial genomes, have been identified. Bacteria are the most useful organisms for identifying essential genes from their specific characteristics: small genomes, rapid growth, and species that are easy to manipulate genetically. Therefore, various investigations on minimal gene sets or minimal genome of bacteria are reviewed, and "the minimal gene set for a viable cell" is discussed.</p>