Last decade, the detection of the CHEK2 c.1100delC mutation allowed for the identification of CHEK2 as a lowpenetrance breast cancer (BC) susceptibility gene [1]. Since then, numerous articles have been published regarding c.1100delC frequency among European hereditary breast cancer (HBC) patients negative for BRCA1/2 (summarized in Desrichard et al. [2]). It seems that c.1100delC shows a Northwest–Southeast frequency gradient in Europe (Fig. 1), but inconsistent results have been reported in Spain. Thus, Osorio et al. [3] fail to detect this mutation in a Spanish cohort comprising 400 non-BRCA1/2 HBC cases and 400 selected controls. On the contrary, Martinez-Bouzas and colleagues [4] detect a 0.93 % prevalence among 214 Basque Country (North Spain) HBC non-BRCA1/2 patients. Similarly, Gutierrez-Enriquez and colleagues [5] report 0.3 % frequency in 331 non-BRCA1/2 HBC families from Basque Country and Catalonia (Northeast Spain). Multiplex ligation-dependent probe amplification (MLPA) routine assays for BRCA1/2 large genomic rearrangements (LGRS), which includes a probe for CHEK2 c.1100delC mutation, led us to identify two (0.36 %) c.1100delC unrelated heterozygous carriers out of 552 Galician HBC families without BRCA1/2 point mutations. As there were no reports on CHEK2 c.1100delC mutation in our population, we screened 1,121 Galician individuals without personal history of cancer, and found one carrier (1/1,121; 0.09 %). Differences between cases and controls were not statistically significant (Fisher exact test p value = 0.23). To our knowledge, this is the largest c.1100delC mutation screening study performed on a Spanish cohort. Our study shows that CHEK2 c.1100delC is present in Galician (Northwest Spain) population and shed light into its distribution in Spanish non-BRCA1/2 HBC patients, pointing to a population substructure for CHEK2, as previously described for other genes [6–8]. Recent reports on non-BRCA1/2 HBC families calculate that c.1100delC heterozygous and homozygous carriers’ odds ratio for BC is 3.29–4.04 [2, 9] and 101.34 [9], respectively. Lifetime BC risk for carriers with an affected first-degree relative has been estimated to be 25 %, but might be higher if the woman has multiple affected relatives [10]. For a carrier with unilateral breast cancer, the risk of a second primary breast cancer is 1 % per year [10]. Moreover, it seems that this mutation is a predictor of adverse prognosis and poor survival (reviewed in Narod et al. [10]). Given this, it is highly recommended to include the screening of CHEK2 c.1100delC mutation in HBC families from populations where this mutation has been described [10]. In our study, we demonstrate the presence of c.1100delC mutation in Galician (Northwest Spain) HBC families, and confirmed the utility of genotyping c.1100delC during the routine screening for BRCA1/2 LGRs, as proposed for CHEK2 low-frequency populations [5]. L. Fachal A. Blanco A. Carracedo A. Vega (&) Fundacion Publica Galega de Medicina Xenomica-SERGAS, Grupo de Medicina Xenomica-USC, CIBERER, IDIS, Santiago de Compostela, Spain e-mail: ana.vega@usc.es