The endogenous locus was disrupted by homologous recombination of a construct containing two engineered stop codons followed by a human growth hormone (hGH) polyadenylation signal and neo. The stop codons were inserted after the first 9 nt encoding the cytoplasmic domain. In spite of the detection of both truncated and elongated transcripts by Northern blot analysis, Western blot analysis showed an absence of cytomplasmic domain peptide sequence in platelet and lung tissue lysates obtained from homozygous mutant mice.