The first recognized epigenetic signal: DNA glucosylation of T-even bacteriophages.

Host-induced modification of phage T2 to T*2 was discovered in 1952. This phenomenon, a reversible alteration in viral host range resulting from a single growth cycle in certain bacterial hosts, is an 'epigenetic' change. In 1963 the chemical basis for the T* modification was shown to be the loss of DNA glucosylation, which resulted from T-even phage growth in cells lacking the glucosyl donor UDPG. Thus, DNA glucosylation of T-even phages was the first recognized epigenetic signal.