An autosomal genome scan for quantitative trait loci (QTL) affecting twinningrate was carried out in the Norwegian Cattle population. Suggestive QTL weredetected on Chromosomes (Chr) 5, 7, 12, and 23. Among these, the QTL positionson both Chr 5 and Chr 23 are strongly supported by literature in the field. Ourresults also confirm previous mapping of a QTL for twinning to Chr 7, butdefinitely suggest a different location of the QTL on this chromosome. The mostconvincing QTL peak was observed for a region in the middle part of Chr 5 closeto the insulin-like growth factor 1 (IGF1) gene. Since IGF1 plays an importantrole in the regulation of folliculogenesis, a mutation search was performed bysequencing more than 3.5 kb of the gene in actual families. The sequencingrevealed three polymorphisms in noncoding regions of the gene that will beimportant in fine structure mapping and characterization of the QTL.