In this study, utilizing single-cell sequencing of BRCA1 mutant breast cancers and time-resolved RNA-seq of BRCA1-deficient normal mammary cells, we revealed that LPs have a trait of higher replication stress compared to other cell populations in the normal mammary tissue, endowing them with the potential for transformation when BRCA1 is deficient, and ELF3 plays a key role in the process. This evidence concerns the gene ELF3 and breast cancer.