Exon 1 was replaced with one in which nucleotide substitutions result in the amino acid substitution of aspartic acid for arginine at position 126 (R126D). This mutation destroys the sialic acid binding site. An FRT-flanked neomyocin resistance cassette with a 5'loxP site was inserted downstream of exon 3. An additional loxP site was inserted downstream of exon 6.