Because variola minor was a less debilitating disease than smallpox, patients were more frequently ambulant and thus able to infect others more rapidly. As such, Variola minor swept through the United States, Great Britain, and South Africa in the early 20th century, becoming the dominant form of the disease in those areas and thus rapidly decreasing mortality rates. Along with Variola major, the minor form has now been totally eradicated from the globe. The last case of indigenous Variola minor was reported in a Somali cook, Ali Maow Maalin, in October 1977, and smallpox was officially declared eradicated worldwide in May 1980. Variola minor was also called white pox, kaffir pox, Cuban itch, West Indian pox, milk pox, and pseudovariola. Genome composition
The genome of Variola major is about 186,000 base pairs in length. It is made from linear double stranded DNA and contains the coding sequence for about 200 genes. The genes are usually not overlapping and typically occur in blocks that point towards the closer terminal region of the genome. The coding sequence of the central region of the genome is highly consistent across orthopoxviruses, and the arrangement of genes is consistent across chordopoxvirusesThe center of the variola genome contains the majority of the essential viral genes, including the genes for structural proteins, DNA replication, transcription, and mRNA synthesis. The ends of the genome vary more across strains and species of orthopoxviruses. These regions contain proteins that modulate the hosts immune systems, and are primarily responsible for the variability in virulence across the orthopoxvirus family.
The FLP gene can then be induced selectively, commonly using either the heat shock promoter or the GAL4/UAS system. The resulting clones can be identified either negatively or positively. In negatively marked clones, the fly is transheterozygous for a gene encoding a visible marker (commonly the green fluorescent protein) and an allele of a gene to be studied (both on chromosomes bearing FRT sites). After induction of FLP expression, cells that undergo recombination will have progeny  homozygous for either the marker or the allele being studied. Therefore, the cells that do not carry the marker (which are dark) can be identified as carrying a mutation. Using negatively marked clones is sometimes inconvenient, especially when generating very small patches of cells, where seeing a dark spot on a bright background is more difficult than a bright spot on a dark background. Creating positively marked clones is possible using the so-called MARCM ("mosaic analysis with a repressible cell marker" system, developed by Liqun Luo, a professor at Stanford University, and his postdoctoral student Tzumin Lee, who now leads a group at Janelia Farm Research Campus. This system builds on the GAL4/UAS system, which is used to express GFP in specific cells. However, a globally expressed GAL80 gene is used to repress the action of GAL4, preventing the expression of GFP. Instead of using GFP to mark the wild-type chromosome as above, GAL80 serves this purpose, so that when it is removed by mitotic recombination, GAL4 is allowed to function, and GFP turns on.