Strain R6 is the Streptococcus pneumoniae isolate from which genomic DNA was obtained for sequencing. The original progenitor of R6 was the serotype 2 strain, D39, a clinical isolate obtained in 1916. Strain D39 is naturally competent for transformation, and because of its virulence, continues today to be employed in models of pneumococcal infection. Strain R36, a rough, unencapsulated mutant of D39, was obtained in 1934 following serial passage in the presence of anti-type 2 capsular antibody. Subsequently, in the 1940s, a transformable, R36A, and a non-transformable, R36N, form of R36 were obtained. The strain used for genomic sequencing, R6, is a direct descendent of R36A and has been employed for several decades as a transformable, unencapsulated prototype of S. pneumoniae for numerous types of physiological, biochemical and genetic studies. It very likely ranks as the most thoroughly investigated of all S. pneumoniae strains. The major, and currently only, conclusively documented genetic difference between strain D39 and the unencapsulated derivatives of it is a 7.5 kb deletion of a contiguous segment of the chromosome that originally encoded nine of the type 2 capsule genes.