The membrane is hybridised overnight at 42◦C in a
water bath with gentle agitation. The stringency of posthybridisation washes should be determined experimentally
for each particular probe. Membranes are washed sequentially, starting in 2 × SSC and 0.1% SDS three times for
10 minutes at room temperature, then twice for 15 minutes
at 65◦C. Membranes hybridised with homologous, long
(longer than 300 bp) probes will probably require stringent washing conditions (i.e., lower salt concentrations);
the salt concentration can be reduced by using 0.1 × SSC
and 0.1% SDS. High stringency washes can be omitted if the
membranes are judged to be washed sufficiently. This is determined by monitoring the membranes with a hand-held
Geiger-Muller counter (1 ¨ −5 counts per minute above background, depending on calibration of the monitor). Washed
membranes are left damp and wrapped in SaranTM wrap,
placed in an autoradiography cassette containing intensifying screens with X-ray film (HyperfilmTM) and exposed at
−70◦C, for between 12 hours and 5 days, depending on the
intensity of the signal.