TTC3 and syphilis: Among the treponemal strains used in this study (Figure 1), four alleles were found at the tprD locus, which include the reference tprD allele (found in the syphilis Nichols, Chicago, and Bal73-1 strains), and the tprD2 allele (found in the syphilis strains MexicoA, Sea81-4, Bal3, and UW249) which encodes the TprD2 protein (35).