Using CRISPR/Cas9, we then knocked out Cmas (the mouse homolog of CMAS) in 4T1 and 6TD1, which are metastatic mouse mammary tumor cell lines from different backgrounds, and find that orthotopically injected Cmas-KO cells have decreased metastatic capacity in vivo, confirming the importance of sialic acid metabolism in breast cancer metastasis. This evidence concerns the gene CMAS and breast carcinoma.