From the NIH Office of Biotechnology Activities (OBA): http://oba.od.nih.gov/oba/index.html

NIH Guidelines Revised to Exempt Most Experiments Involving the Breeding of Transgenic Rodents Housed Under BL1 Conditions - Under the revisions to the NIH Guidelines for Research Involving Recombinant DNA Molecules effective January 19, the breeding of transgenic rodents that may be housed under BL1 containment conditions is exempt from the NIH Guidelines with the exception of:

• Breeding experiments involving transgenic rodents that contain more than 50 percent of the genome of an exogenous eukaryotic virus from a single family, in order to prevent inadvertent reconstitution of an exogenous virus in the resultant transgenic rodent; and
  
• Breeding experiments in which the transgenic rodent's transgene is under the control of a gammaretroviral long terminal repeat (LTR), in order to address the small risk of recombination with endogenous retroviruses which could potentially result in mobilization of the transgene via a replication-competent mouse retrovirus.

The above two types of experiments must still be registered with, and eventually approved by, an IBC under Section III-E of the NIH Guidelines.

A full description of these changes can be found in the January 19, 2011 Federal Register (76 FR 3150)