Structure-specific DNA endonuclease that specifically cleaves single-stranded DNA and 3' overhang DNA. Contributes to the control of DNA double-strand break repair choice by antagonizing BRCA1-dependent homologous recombination (HR) and promoting non-homologous end-joining (NHEJ). Recruited to the single-stranded DNA ends by SHLD2 and cleaves the 3' exposed DNA ends, therefore inhibiting DNA end resection (necessary for HR) and promoting DNA end protection (necessary for NHEJ). . This is the function of Ensembl gene identifier ENSG00000034533 (ASTE1, asteroid structure-specific endonuclease 1).