A hygromycin resistance cassette replaced the first 42 codons of exon 31, the exon 31 splice acceptor site, and approximately 2 kb of intron 30. This allele is identical to Nf1tm1Tyj except that a hygromycin cassette was used instead of a neomycin cassette. This targeting event occurred in ES cells already containing the Nf1tm1Tyj allelle. (J:58876)