Here we tell a 20-year long story. It began with an easily overlooked DNA degradation (Dnd) phenomenon during electrophoresis and eventually led to the discovery of an unprecedented DNA sulfur modification goveed by five dnd genes. This unusual DNA modification, called phosphorothioation, is the first physiological modifica-tion identified on the DNA backbone, in which the nonbridging oxygen is replaced by sulfur in a sequence selective and stereo-specific manner. Homologous dnd gene clusters have been identified in diverse and distantly related bacteria and thus have drawn immediate attention of the entire microbial scientific community.Here, we summarize the progress in chemical, genetic,enzymatic, bioinformatical and analytical aspects of this novel postreplicative DNA modification. We also discuss perspectives on the physiological functions of the DNA phosphorothioate modification in bacteria and their implications.