To obtain an animal model for studying the role of the GH receptor (GHR) in growth and development, we analyzed a sex-linked dwarf (SLD) chicken strain (Leghorn) which exhibits phenotype similarities with a human genetic growth disorder, an autosomal recessive GH resistance condition (Laron dwarfism). Having previously demonstrated the responsibility of the human GHR gene in the Laron phenotype, we focused our analysis on the corresponding gene in SLD chickens. Sequencing of the whole coding region of the chicken GHR cDNA identified a G-to-T transversion segregating with the SLD phenotype and generating an isoleucine instead of a serine at position 199 within a highly conserved region close to the junction between the extracellular and transmembrane domains. This defect involves the last invariant amino acid of the WS-like motif (amino acid sequence WSXWS) common to all members of the cytokine receptor superfamily. Transfection of a mutated GHR cDNA containing this mutation into eukaryotic cells led to the synthesis of a receptor protein that displayed impaired plasma membrane expression and binding activity. These data define the molecular basis for the SLD phenotype and identity this strain as an interesting model for studying Laron dwarfism in humans; this animal model may also represent a system in which therapeutic strategies to promote growth can be evaluated. Finally, the nature of the molecular defect identified provides direct evidence for the functional importance of the WS motif in GHRs and related receptors.