Some transformable bacteria have acquired target-mediated antibiotic resistance by horizontal genetic exchange of fragments of chromosomal genes. The resistant strains express variants of the antibiotic target that are metabolically active but exhibit a lowered affinity for the antibiotic. The alleles encoding these resistant proteins are mosaics comprising DNA derived from the host and other bacteria, often members of a different species. Examples include penicillin-resistant penicillin-binding proteins (PBPs) in Streptococcus pneumoniae and the pathogenic Neisseria species and sulfonamide-resistant dihydropterate synthase in Neisseria meningitidis. Distinct mosaic alleles encoding antibiotic resistance have arisen on multiple occasions, indicating the mobility of chromosomal genes in these species. Mosaic genes can arise at any chromosomal locus, and S. pneumoniae organisms with high-level penicillin resistance have acquired mosaic PBP genes at three bacterial bpb loci. Furthermore, horizontal genetic exchange permits movement of alleles among bacterial lineages, increasing the opportunities for the spread of antibiotic resistance.