Plots of nonessential (NE) gene number versus genome size and total genes in Mycoplasma species and reduced strains. NE genes were determined by transposon mutagenesis in Mycoplasma genitalium (580 kb), Mycoplasma pneumoniae (816 kb), Mycoplasma pulmonis (963 kb), Mycoplasma mycoides JCVI-syn1.0 (1080 kb), M. mycoides JCVI-syn2.0 (576 kb), and M. mycoides JCVI-Syn3.0 (531 kb). (A) Plotting NE genes versus genome size yields an extrapolated value of 423 kb for the minimal genome size when NE = 0. (B) Plotting NE genes versus total genes yields an extrapolated value of 413 genes when NE = 0.