In situ hybridization
For in situ hybridization, probes were labelled with digoxigenin-11-UTP using the nucleic acid labelling kit (Boehringer). The TOFL cDNA in pGEM-T vector was amplified by PCR with forward and reverse universal primers, and used as a template to synthesize digoxigenin-labelled sense and antisense RNA with T7 and SP6 RNA polymerases, respectively. Sample preparation and hybridization were performed as described by Huijser et al. (1992) .