A PGK neomycin cassette was inserted into exon 4. This sequence encodes the second alpha helix of the protein. RT-PCR analysis confirmed a lack of wild-type transcript in homozygous mutant mice. Western blotting of pituitary gland extracts revealed the presence of a unique immunoreactive ~10-kDa truncated protein product in homozygous mutant mice. Disrupted prolactin synthesis was confirmed by biological assays.