Our study involved two extended familial atypical multiple mole melanoma (FAMMM) kindreds wherein a sufficient number of informative, high genetic risk, and affected patients enabled collection of pertinent tissue samples (normal skin/fibroblasts and atypical nevi/melanocytes) for cytogenetic analysis, and peripheral blood lymphocytes for DNA usage for linkage studies. We observed marked chromosome instability, as evidence by increased frequencies of cells with chromosomal rearrangements (translocations, deletions, and inversions) in cell cultures from atypical nevi and normal skin. There was no evidence of linkage of the FAMMM disease locus to any of the markers for the short arm of chromosome 1p in these two families. Well-characterized FAMMM kindreds provide an opportunity for biomarker investigations for elucidating heterogeneity and, ultimately, improving cancer control.