Map of the ENPP1 locus. The 25 ENPP1 exons (boxes) are numbered from left to right according to the direction of transcription. The filled boxes constitute the coding region. The open box represents the long 3' untranslated region. Vertical arrows point to the polymorphic sites analyzed in this study. M06NR1A is located some 46 kilobase pairs (kb) upstream of the promoter. The other intermarker distances may be taken from the graph, which is drawn to scale. DNA markers with alleles that were found to be significantly associated with hand OA (see Tables 3 and 4) are marked by arrowheads.