Genomic instability is a hallmark of cancer. Defects in DNA repair fuel genomic instability and we still lack mechanistic details of these DNA repair pathways in an organismal context. A closer investigation of patients with human genome instability syndromes provides a rich and unparalleled opportunity to uncover novel pathways, as demonstrated by the identification of germline ATM mutations as a cause of Ataxia- telangiectasia. Thus, my research concept aims to contribute to a deepened functional understanding of genome maintenance and DNA repair in cancer. In one stream of my research I analyze patients with - as of yet - uncharacterized genome instability syndromes, drawing from a cohort of > 300 clinically annotated and trio-whole-exome-sequenced patients with clinical features of genome instability. Emphasis is placed on the double-strand break (DSB) repair pathway choice of homologous recombination repair (HRR) vs. non-homologous end joining (NHEJ) due to its highly relevant translation of treatment rationale in cancer. Moreover, I also utilize a systematic approach to identify novel factors in the DSB repair pathway choice by the use of genetic drop out screens in combination with the SeeSaw DNA repair fluorescent reporter assay. The identification of novel HRR defective pathways within this screen will pave the way for a treatment rationale with PARP1 inhibitors in cancer.