Trapped gene association determined by genome coordinate overlap and strand. The gene trap inserted within the coding region of ABD CH, disrupting the Dst 1 and 2 actin-binding domain-containing isoforms transcribed from upstream promoters and generating a Dystonin-lacZ fusion product. RT-PCR analysis indicates that these isoforms are either absent or barely detectable in homozygotes. In addition, the F1Ex system relies on the sequential use of the Cre and FLP recombinases for the conditional inactivation and reactivation of the gene trap alleles. The first site-specific recombination using FLP recombinase reverses the gene trap cassette from the original mutagenic orientation to a non-mutagenic, inverted orientation. Subsequent recombination using cre recombinase leads to inversion of the transgene back to the mutagenic direct orientation.