Identification of genes underlying quantitative trait loci (QTLs) represents a formidable task. Recent advances in the annotation of the mouse genome and the availability of newly developed mouse strains should boost gene discovery. Kato and colleagues report the localization of a QTL for collagen type I deposition in kidneys on mouse chromosome 2, representing the first step for the identification of a gene involved in interstitial fibrosis, an important pathway in progression to end-stage renal failure in many kidney disorders.