Exon 7 was disrupted by the insertion of a neomycin selection cassette that introduced stop codons into all three reading frames. A binding assay of homozygous bone marrow cells showed that high affinity binding of the colony stimulating factor was ablated. Binding of interleukin 3 was unaffected in cells isolated from mutant mice, indicating that the homologous interleukin 3 receptor was not affected by the targeting event.