As part of a genome scan, ESTs derived from mammary gland tissue of a lactatingcow were used as candidate genes for quantitative trait loci (QTL), affectingmilk production traits. Resource families were genotyped with 247 microsatellitemarkers and 4 polymorphic ESTs. It was shown by linkage analysis that one ofthese ESTs, KIEL_E8, mapped to the centromeric region of bovine Chromosome (Chr)14. Regression analysis revealed the presence of a QTL, with significant effecton milk production, in this chromosome region, and analysis of variance showedno significant interaction of marker genotype and family. The estimatedsignificant differences between homozygous marker genotypes were 140 kg milk,-5.02 kg fat yield, and 2.58 kg protein yield for the first 100 days oflactation. Thus, there was strong evidence for a complete or nearly completelinkage disequilibrium between KIEL_E8 and the QTL. To identify the biologicalfunction of KIEL_E8, we extended the sequence for 869 bp by 5'-RACE. A 560-bpfragment of this shows a 90.9% similarity to a gene encoding a cysteine- andhistidine-rich cytoplasmic protein in mouse. Although such a protein may have aregulatory function for lactation and a linkage disequilibrium between the ESTmarker and the QTL has been observed, it remains to be elucidated whether theyare identical or not. Nevertheless, KIEL_E8 will be an efficient marker toperform marker-assisted selection in the Holstein-Friesian population.