In addition to the three alleles commonly responsible for the protein polymorphism of human deoxyribonuclease I, a mutation encoded by a fourth allele, DNASEI*4, was detected by isoelectric focusing. All 8 exons covering the entire open reading frame of the human DNase I gene were amplified by the polymerase chain reaction and subjected to direct sequencing. Only one nucleotide substitution, a C-to-G transition (CAG → GAG), in the codon for amino acid 9 of the mature enzyme was found. This substitution resulted in the replacement of Gln with Glu (Q9E).
