We report here results of a linkage analysis of a marker in 35 families in which the proband had premenopausal bilateral breast cancer. This group is of particular interest given their high family risk and the question of etiological heterogeneity. Probands were ascertained from cancer registries in Los Angeles County and Connecticut and major hospitals in Montréal and Québec. Assuming no residual heterogeneity and summing lod scores over all families, we obtained strong evidence against tight

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... inkage (e.g., lod score at theta = 0.000001 is -3.39). To address the issue of heterogeneity, we performed admixture and predivided sample analyses. Using an admixture model we were able to reject the hypothesis of no linkage versus that of linkage with homogeneity (P = 0.045). However, we were unable to reject the hypothesis of no linkage versus linkage with heterogeneity (P = 0.119) or to distinguish between linkage with homogeneity and linkage with heterogeneity (P = 0.500). Predivided sample analyses based upon age of onset, pathological characteristics, time between diagnoses of the breast cancers in each bilateral proband, and the span of ages at diagnoses within a family did not discriminate between apparently linked and unlinked families.