These data provide first in vivo evidence that tyrosyl phosphorylation of PAK1 plays a significant role in PRL-induced breast cancer cell motility and metastasis, as only cells overexpressing PAK1 WT, but not phospho-tyrosine-deficient PAK1 Y3F, were able to migrate from the primary tumor to the lungs. The gene discussed is PAK1; the disease is breast carcinoma.