Here, we show in a genetic system tolerating BRCA1 deficiency that HR impairment results in three different classes of mutations that are also observed in HR-deficient tumour cells: (i) small deletions with overrepresentation of micro-homology at the junction, (ii) duplications of ~10 kb that are located immediately adjacent to their counterpart, (iii) single nucleotide variants. The gene discussed is BRCA1; the disease is neoplasm.