(A) The genomic organizations of the wild-type and targeted sam68 alleles after homologous recombination are depicted. The location of the DNA fragment used as a probe for the Southern blot analysis is shown, as well as the sizes of the two BglII fragments detected for wild-type and targeted sam68 alleles. The targeted allele replaces exon 4 and part of exon 5 of sam68 with a PGK-neomycin cassette.