The host is the organism into which a gene from another organism is transplanted. The guest gene is carried by a vector, which is a larger DNA molecule, such as a plasmid, or a virus into which that gene is genetically engineered and which then propagates in the host.NIH Guidelines under Appendix E. - Certified Host-Vector Systems contains a list of derivatives host-vector systems previously classified as Host-Vector 1 Systems or Host-Vector 2 Systems, they are: Bacillus subtilis, Saccharomyces Cerevisiae, Escherichia coli, Neurospora crassa, Streptomyces, and Pseudonomas putida.