A cDNA encoding cre recombinase with an N-terminal nuclear localization signal (nls) was inserted immediately before the translation initiation codon in the gene. Under the control of this promoter, Cre activity is detected by embryonic day 12.5; Cre mediated recombination was observed in dorsal telencephalon, neocortex, hippocampus, and olfactory bulb. An Frt flanked Neo cassette was retained in this allele