Hes1F 59-GAGAGGCTGCCAAGGTTTTT-39 Hes1R 59-GTAATACGACTCACTATAGGGTCAAA- TAAACTTCCCCAAAGGA-39 The following riboprobes were prepared from plasmid tem- plates: Crabp1 [34], Fgf10 [3], Isl1 [35], PlexinA2 [36], lacZ [3]. Riboprobes for Opa1, 953002-Rik, Gm1968, Hey1 and Hey2 were generated using T7 RNA polymerase from PCR amplified templates. Primer details are avaliable on request. For each experiment a minimum of two embryos per genotype were scored. Fluorescent in situ hybridization Fluorescent in situ hybridization was carried out as described [37]. Briefly, metaphase spreads were prepared from male mice carrying the T55 transgene. Concanavalin A-stimulated splenic cells were cultured for 72 hours with 5-BrdU added for the final 6 hours of culture (60 mg/ml of medium) to ensure chromosomal R-banding. The biotinylated lacZ probe was mixed with hybridization solution at a final concentration of 50 mg/ml and used at 200 ng per slide after competition of aspecific repetitive sequences. 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