Nucleotide sequences of the membrane exons of human immunoglobulin pseudo-gamma gene and gamma 1 chain cDNA were determined and compared. The sequences of the membrane exons of the pseudo-gamma gene had no defective mutations in the coding regions or the exon-intron boundaries. The translated amino acid sequence was highly homologous to that of the active gamma 1 gene, with only one substitution, at the 16th amino acid of the M2 exon (valine/isoleucine). Our findings together with previous data strongly support the idea that the absence of product of the pseudo gamma gene is attributable to deletion of 5' flanking regions responsible for class switch recombination (S region) and transcription of the germline CH gene (I region). The S and I regions are, therefore, likely to be indispensable for the expression of the immunoglobulin gamma, epsilon and alpha genes.