Using a PCR strategy based on an initial set of 15 couples of primers designed from the known cDNA sequence, we identified 18 introns in the human RET proto-oncogene and sequenced the corresponding 5' and 3' exon-intron junctions. This approach was successful in locating all the introns contained in fragments short enough to be amplified by PCR. Thus 19 exons were identified which, together with the previously reported exon subjected to alternative splicing, brings the total number of RET exons to 20. This information is relevant for the screening of recently reported missense mutations of RET which cause Multiple Endocrine Neoplasia 2A (MEN2A) and for the search of additional point mutations of the same gene which might cause two other neural crest disorders, MEN2B and Hirschsprung disease, mapping in the same region as MEN2A.