Fig. 2. NFH–Cre transgene, temporal expression. (A) The NFH–Cre construct containing the Cre-coding region (grey box) inserted into exon 1 (E1) of the murine NFH gene upstream of the NFH start codon (ATG). Black box, SV40 polyadenylation signal; white boxes, NFH exons. NLS/ATG, nuclear localization signal and start codon of Cre gene. (B) Southern blotting of EcoRI-digested DNA from the brains of two tg46–Cre 22 (lanes 1 and 2), four tg46 (lanes 4–6 and 11) and two tg37–Cre 22 (lanes 8 and 9) mice. DNA from a Prnp0/0 mouse brain was loaded in lane 10. Hybridization of probe with unrecombined 3.2 kb and recombined 2.2 kb fragments is seen in double transgenic animals, the proportions of each fragment are shown beneath the image. There is no recombination in tg46 mice. (C) A histogram showing the time course of onset of Cre-mediated recombination of the MloxP locus. DNA from brains from three or more tg46–Cre 22 or tg37–Cre 22 mice or embryos were analysed for recombination by Southern blotting as above. E, embryonic day; P, post-natal day; w, weeks of age. Mean values for the number of mice indicated are depicted. For all time points up to 12 weeks, n = 3.