A Tkneo selection gene flanked by loxP sites inserted in between the tandemly arranged exons 5a and 5b and exon 6 via homologous recombination, disrupting the developmental regulation of alternative splicing between exons 5a and 5b. In homozygous mutants, RT-PCR confirmed that expression of the two isoforms is altered such that the expression of the a isoform is increased and the b isoform is decreased. (J:94005)