The study built upon existing sib-pair methods for multipoint mapping of Quantitative Trait Loci (QTL)s. It greatly expanded upon the statistical methodology to identify and localize QTLs for complex traits by merging sib-pair methods with a powerful twin design which used phenotypic data on monozygotic (identical) and dizygotic (fraternal) twins to resolve phenotypic variability into genetic and non-genetic components, and partition human quantitative genetic variation into effects due to loci on specific chromosomal regions. This quantitative methodology was applied to risk factors for cardiovascular disease -- one of the most pressing health problems in Western society. Population-based samples of Dutch, Swedish, and Australian twins had been identified in previous studies, phenotypic measures on lipids, lipoproteins, and other important cardiovascular risk factors had been obtained, and blood had been banked in a valuable resource which was available for use in this study. In 500 DZ twin pairs, a series of 175 highly polymorphic microsatellite markers were detected using an automated process for detecting fluorescent signals. These data were analyzed using the new statistical methods both to confirm the effects of a series of candidate loci and to test for the effects of previously unknown QTLs.