A neo cassette with a 5' loxP site was inserted upstream of exon 3 and an additional loxP site was inserted downstream of exon 3. Transient cre expression in ES cells was used to removed the neo cassette and exon 3 causing a frame shift and premature termination of translation. The absence of the short and long forms of the protein was confirmed by western blot analysis on brain and heart extracts. (J:140030)