Infrastructure matters - as the cost and difficulty of commonly used techniques fall, more progress is accomplished more rapidly. Here is a promising development in stem cell research infrastructure: "Biologists have developed an efficient way to genetically modify human embryonic stem cells. Their approach, which uses bacterial artificial chromosomes (BACs) to swap in defective copies of genes, will make possible the rapid development of stem cell lines that can both serve as models for human genetic diseases and as testbeds on which to screen potential treatments. ... This will help to open up the whole human embryonic stem cell field. Otherwise, there's really few efficient ways you can study genetics with them. ... BACs are synthesized circles of human DNA, which bacteria will replicate just like their own native chromosomes. Commercially available BACs can be modified within bacterial cells to insert altered copies of specific genes. Once the modified BACs are introduced into human cells, they will sometimes pair up with a matching segment of a human chromosome and swap segments of DNA, a process called homologous recombination. ... Using BACs, the team was able to substitute modified genes in 20 percent of treated cells. Standard methods of genetic modification typically achieve modification in fewer than one percent of cells."