A neurofibromatosis type-1 (NF1)-related locus on human chromosome 21 has been characterized. A detailed genomic mapping performed by yeast artificial chromosome (YAC) dot-hybridization revealed that the NF1-related locus is close to the sequence-tagged site (STS), D21S329 (G52E12), which is located on the proximal region of 21q11.2. Sequence analysis showed that this locus seemed to be conserved to the NF1 gene only in several partial regions. Two exon-like segments corresponding to exons 8 and 9 of NF1 were found, in addition to two previously found fragments corresponding to exons 7 and 11. Other exon-like segments were not found in the region so far sequenced. Comparing these homologous segments with the NF1 cDNA, a 2-bp deletion appeared in exon 8 of the locus, resulting in the existence of stop codons in all reading frames. In addition, there were no amplified fragments derived from the related locus by reverse transcription-PCR. Thus, our results suggest that the NF1-related locus on chromosome 21 is a nonprocessed pseudogene.