Hence, species-specific genes in Leishmania are conserved in isolates from within the same complex (L. donovani vs. L. infantum) and between strains of the same species isolated from different geographical locations (e.g., L. donovani BPK206/0 from Nepal and L. donovani LV9 from Ethiopia) or similar geographical locations (e.g., L. mexicana U1103 from Guatemala and L. mexicana M379 from Belize). The majority of the L. donovani complex species-specific genes are uncharacterized, and for most, their transgenic expression in L. major failed to identify a phenotype correlating with visceral infection in mice (Zhang et al. 2008; Zhang and Matlashewski 2010). An exception was LinJ.28.0340, a L. infantum–specific gene apparently important for survival of axenic amastigotes (Zhang and Matlashewski 2010). The finding that unique genes are present in both L. infantum and L. donovani suggests that these sequences play an important role in the potential for visceralization associated with the L. donovani complex. However, other structural and functional components of the genome, such as gene copy number (Fig. 2) and differential gene expression (Depledge et al. 2009), are also likely to be important.