These mice possess loxP sites on either side of exon II (also called exon 3, when the first non-coding exon is included in the numbering scheme) of the targeted gene. Mice that are homozygous for this allele are viable and fertile. Sequence analysis revealed a single base deletion mutation in the first loxP site. When these mutant mice are bred to mice that express Cre recombinase, recombination efficiency is weak and resulting offspring may not have exon 2 deleted in cre-expressing tissue(s). When primary cell cultures derived from this strain (cortical neurons) are infected with lentiviral particles expressing the Cre recombinase, exon II is deleted. These mice are are an effective tool for achieving cre recombination but only in an ex vivo context. The Jackson Laboratory 2024, used with permission.