Results: While primary tumor growth did not differ betweenEfna1+/+,Efna1+/-, andEfna1-/- mice, lung metastasis and primary tumor recurrence were significantly decreased in knockout mice.Efna1-/- mice had reduced lung colonization of 4T1 cells compared toEfna1+/+ littermate controls as early as 24 hours after tail vein injection. Here, EFNA1 is linked to neoplasm.