3-M syndrome is a primordial growth disorder caused by mutations in CUL7, OBSL1 or CCDC8. 3-M patients typically have a modest response to GH treatment, but the mechanism is unknown. Our aim was to screen 13 clinically identified 3-M families for mutations, define the status of the GH-IGF axis in 3-M children and using fibroblast cell lines assess signalling responses to GH or IGF1. Eleven CUL7, three OBSL1 and one CCDC8 mutations in nine, three and one families respectively were identified, those with CUL7 mutations being significantly shorter than those with OBSL1 or CCDC8 mutations. The majority of 3-M patients tested had normal peak serum GH and normal/low IGF1. While the generation of IGF binding proteins by 3-M cells was dysregulated, activation of STAT5b and MAPK in response to GH was normal in CUL7(-/-) cells but reduced in OBSL1(-/-) and CCDC8(-/-) cells compared with controls. Activation of AKT to IGF1 was reduced in CUL7(-/-) and OBSL1(-/-) cells at 5 min post-stimulation but normal in CCDC8(-/-) cells. The prevalence of 3-M mutations was 69% CUL7, 23% OBSL1 and 8% CCDC8. The GH-IGF axis evaluation could reflect a degree of GH resistance and/or IGF1 resistance. This is consistent with the signalling data in which the CUL7(-/-) cells showed impaired IGF1 signalling, CCDC8(-/-) cells showed impaired GH signalling and the OBSL1(-/-) cells showed impairment in both pathways. Dysregulation of the GH-IGF-IGF binding protein axis is a feature of 3-M syndrome.