The remaining 14 genes are claimed to be strong candidates for ancient gene transfers from proteobacteria to nuclear genomes. Because no mtDNA-encoded homologs of these genes are currently known, however, the formal possibility exists that some of them (for instance, those encoding mitochondrial heat-shock proteins) have arisen by lateral gene transfer at a separate time from the mitochondrial endosymbiosis [37]. Strictly speaking, we can only be certain of the 64 protein-coding genes of assigned function in R. americana mtDNA [19] as deriving directly from the mitochondrial endosymbiont.