This study reports the first demonstration of specific mutations leading to human adenine phosphoribosyltransferase (APRT) deficiency. The molecular basis of the deficiency was investigated by determining the sequence of both alleles of a patient with a complete deficiency in APRT activity. A trinucleotide deletion, corresponding to phenylalanine on the deduced amino acid sequence, was confirmed on one allele. A single nucleotide insertion, immediately adjacent to the splice site at the 5' end of the fourth intervening sequence, was confirmed on the other allele. This insertion lead to aberrant splicing, as was demonstrated by the absence of exon 4 in the complementary DNA sequence and by altered RNase mapping analysis of the abnormal messenger RNA.