Scienceisawonderfullyresilientventure.Therearenewandinterestingdevelopmentstoreportineachrevisionofthisbook，andthisrevisionincludesmuchupdatedmate-rialtoaccountfornewfindingsinmolecularresearch.ThegeneralorganizationofmaterialinthiseditionhasbeenrevisedalongthesamelinesasEssentialGenes，makingiteasiertousethetwobooksinconjunction.Withincreas-ingsizebecomingaproblem，thecontenthasbeenmoresharplyfocusedongenesandtheirexpressionbyeliminat-ingthechaptersdealingwiththeconsequencesofgeneex-pressionforcellbiology.Strikingchangesoccurinthefirstpartofthebook，dealingwithgenomes，resultingfromthesuccessofmanygenomesequencingprojects.Theimpor-tanceofRNAasaregulatorhasbecomeincreasinglyevi-dentandnowcanbeseentoextendacrossalllevelsofgeneexpressioninbothprokaryotesandeukaryotes.Somewhatofamissinglink，itcastsfurtherlightonhowthecurrentapparatusforgeneexpressionmusthaveevolvedfromtheearlyRNAworld.