Fifteen patients with selective IgG1 deficiency were screened for immunoglobulin H chain C region locus (IGHC) gene deletions and three deletion haplotypes were found: del Gl, delGI G4 and del G4. These haplotypes, together with four deletion haplotypes described by us previously (del G1 (NY), del G1 (VIT) del G1-G2 (NY) and del G2-G4 (HJE)). were further characterized using pulsed-field gel electrophoresis (PFGE) to determine the physical extent of the deletions. The MluI fragment sizes confirmed the deletions, although the deduced sizes of the most extensive deletions indicated that material had been inserted into the locus.