Germline mutations of the breast tumor suppressor gene BRCA1 predispose women to breast and ovarian cancers. However, loss-of-function mutations of mouse Brca1 results in recessive embryonic lethality, which obscures the functions of BRCA1 in breast cancer formation. Cre-loxP-mediated tissue-specific knockout was employed to overcome this obstacle. We found that the presence of a ploxP-neo-loxP cassette in intron 10 of Brca1 resulted in severe interference with gene expression. The neo cassette was deleted in either embryonic stem cells or mice to generate the neo-less conditional knockout allele. Finally, we performed functional analysis of mammary tumorigenesis in Brca1 conditional knockout mice. The methods to generate and analyze these Brca1 conditional knockout mice are described in this chapter.