The GIPC1 gene product promotes clustering of some transmembrane receptors, including those involved in carcinogenesis, and protects them against ubiquitin-dependent degradation. The 5' untranslated region of GIPC1 contains a polymorphic trinucleotide CGG repeat, which has not been characterized earlier. In the present study, we have carried out comparative analysis of the allele and genotype frequencies of this repeat in 129 samples of breast cancer (BC), 58 samples of non-small cell lung cancer (NSCLC), and 215 samples of healthy donors. The CGG repeat in the 5' untranslated GIPC1 gene region was shown to be highly polymorphic and represented by at least eight alleles. Alleles CGG10-13 were major, occurring at frequencies of 22, 41, 27, and 9%, respectively; the total frequency of the remaining alleles was approximately 1%. Heterozygosity of the CGG repeat was 0.70. Allele CGG12 was shown to be associated with high risk of developing NSCLC (alpha = 0.05).