GH insensitivity (GHI) is an autosomal recessive disorder caused by defects in the GH receptor (GHR). In a 17-yr-old female with severe short stature and biochemical features of GHI, sequencing of GHR gene revealed a compound heterozygosity for two novel mutations: C83X and a G deletion at position 1776 (1776del). 1776del is predicted to result in GHR truncation to 581 amino acids with a nonsense sequence of residues 560-581. To clarify the effect of 1776del on GHR function, wild-type GHR, GHR-1776del, and two additional GHR mutants, GHR-L561X (stop codon at site of the 1776del) and GHR-I582X (translation termination in GHR-1776del) were transiently expressed in CHO cells. After incubation with recombinant human GH, GHR-1776del showed lower signal transducer and activator of transcription 5 (STAT5)-mediated transcriptional activation ( approximately 50%, P < 0.05), as well as STAT5 Tyr694 phosphorylation (P < 0.05) compared with wild-type GHR, whereas GHR-L561X and GHR-I582X showed normal STAT5 phosphorylation and transcriptional activity. In contrast, all vectors produced similar effects on STAT3-mediated transcriptional activation. In conclusion, this novel GHR-1776del mutation in a classical GHI patient illustrates an important mechanism of impaired GHR-STAT5 but intact GHR-STAT3 signaling. This effect might result from interference of C-terminal nonsense sequence in mutated GHR with STAT5 docking to upstream tyrosine residues.