Heterozygous mice. The genetic background of these mice was stated to be on a ">=75%" C57BL/6 genetic background. This background also contained some 129S4/SvJae and possibly some other strain or strains.
These mice carry the transgene and are heterozygous for the targeted allele. The genetic background of these mice was a mixture of at least C57BL/6, DBA/2, and 129S4/SvJae. Other strains may also have been involved.