An FRT flanked neo cassette was inserted in intron 5 and exon 5 was flanked 5' and 3' with loxP sites 532 bp upstream and 934 bp downstream. Aberrant splicing due to the use of cryptic splice sites results in a hybrid mRNA containing an additional portion of intron 5 and the coding sequence of neo. Wild-type RNA is reduced in this hypomorphic allele.