We compared the sequence of CD1214 obtained from our lab strain 630Derm [38] to that of the published C. difficile 630 genome [42] . Strain 630Derm is a spontaneous erythromycin sensitive strain, which is commonly used in mutagenesis studies and was obtained by serial passaging of strain 630 [33, 38] . The 630Derm spo0A sequence (Genbank accession no JX050222) was derived from the expression plasmids constructed for this study, and confirmed in a whole genome sequence of strain 630Derm generated in our lab (data not shown). We found that 630Derm spo0A contains an 18 base pair direct repeat, resulting in a 6 amino acid (NVGNIE) duplication compared to the published reference sequence. The duplication maps to a region of the protein with relatively low sequence conservation (hinge), flanking the highly conserved DNA binding domain ( Fig. 2A  and B) . We verified the absence of this duplication in strain 630 by PCR (Fig. 2C ) as well as sequencing from the chromosomal DNA of C. difficile 630 (data not shown), to rule out an error in the original genome sequence and to demonstrate that the difference in size of the PCR product was specific to the 18 bp insertion. In addition, we checked several other strains of PCR ribotypes 12 (to which 630 and 630Derm belong) by PCR, but the duplication was found to be unique to 630Derm among the isolates tested (data not shown).