A mouse embryonic tissue Northern blot filter (Seegene, Rockville, Maryland, United States) was hybridized with a Sox6 probe generated by RT-PCR (nucleotides 1353–1927) and labeled with [α-32P]dCTP, by random primer labeling (RediprimeII; Amersham Biosciences, Buckinghamshire, England, United Kingdom). The hybridization was performed in phosphate buffered 7% SDS hybridization solution. Blots were washed with 0.2× SSC, 1% SDS at 60 °C prior to exposure to X-ray film (Kodak, Rochester, New York, United States) at −80 °C for 6 d.