Complex traits. Traits which are of the most interest to us belong to the category of complex traits. Complex traits are controlled by many interactive components, including developmental-environmental factors, and many genes. In the biomedical field most complex traits, such as body weight, cancer susceptibility, diabetes, blood pressure, depression, alcoholism, etc., have heritable components. Historical Background. Focusing on single genes and qualitative traits, Bartlett and Haldane (Bartlett and Haldane, 1935) indicated that the object of an experiment may be to introduce a certain known gene "into a pure line, so as to study its effects against a standard genetic background." In the early 1980s, it was generally thought that the introgression of (unknown) genes for complex traits was limited to qualitatively distinguishable phenotypes. For example, Earl. L. Green (Jackson Laboratory), suggested that "To be successful, the method must be limited to discrete traits or to continuous traits that exhibit clear bimodality in the segregating strains" (Green, 1981) (p.117). The RQI concept. The RQI principle posits that a set of unidentified polygenes (or QTLs) which define the expression of a quantitative trait with continuous distribution can be transferred together to a another genetic background. The transfer can be done for various reasons, e.g., for further genetic analysis (e.g., for gene mapping with inbred quasi-congenic Recombinant QTL Introgression strains), construction of advanced animal research models, which are genetically identical except for a small fraction of their genome which carries the genes responsible for the complex trait difference, development of new breeds of economic value. RQI strains. The gene transfer related work in the Laboratory of Neurobehavioral Genetics began in 1983. Based on the RQI principle, inbred quasi-congenic RQI strains of animals have been constructed for mapping polygenes and for studying the physiological mechanism of complex traits in advanced animal models in which the probability of chance association between phenotypes is small (Vadasz, 1990; Vadasz et al., 1994; Vadasz et al., 1996; Vadasz et al., 1998). The experimental results confirmed the RQI principle that unknown polygenes for complex traits can be introduced into a pure line. Currently, we maintain 100+ RQI mouse strains. The majority of the strains (b5i7 series) is about 97% identical with the C57BL/6By background strain (Vadasz et al., 2000a; Vadasz et al., 2000b). The remaining 3% is distributed in the form of small chromosome segments of BALB/cJ origin.