Testing can be done on only 12 of the original lines, and all approved lines have been cultured in contact with mouse cells, which creates safety issues that complicate development and approval of therapies from these lines.[170] On July 19, 2006, Bush used his veto power for the first time in his presidency to veto the Stem Cell Research Enhancement Act. The bill would have repealed the Dickey–Wicker Amendment, thereby permitting federal money to be used for research where stem cells are derived from the destruction of an embryo.[171]

Immigration
President Bush discussing border security with Secretary of Homeland Security Michael Chertoff near El Paso, November 2005
Nearly eight million immigrants came to the United States from 2000 to 2005, more than in any other five-year period in the nation's history.[172] Almost half entered illegally.[173][unreliable source?]In 2006, Bush urged Congress to allow more than twelve million illegal immigrants to work in the United States with the creation of a "temporary guest-worker program". Bush also urged Congress to provide additional funds for border security and committed to deploying 6,000 National Guard troops to the Mexico–United States border.