MRCKα is a ubiquitously expressed serine/threonine kinase involved in cell contraction and F-actin turnover, which is highly amplified in human breast cancer and part of a gene expression signature for bad prognosis. Nothing is known about the in vivo function of MRCKα. To explore MRCKα function in development and in breast cancer, we generated mice lacking a functional MRCKα gene. Mice were born close to the Mendelian ratio and showed no obvious phenotype including a normal mammary gland formation. Assessing breast cancer development using the transgenic MMTV-PyMT mouse model, loss of MRCKα did not affect tumor onset, tumor growth and metastasis formation. Deleting MRCKα and its related family member MRCKβ in two triple-negative breast cancer cell lines resulted in reduced invasion of MDA-MB-231 cells, but did not affect migration of 4T1 cells. Further genomic analysis of human breast cancers revealed that MRCKα is frequently co-amplified with the oncogenes ARID4B and AKT3 which might contribute to the prognostic value of MRCKα expression. Collectively, these data suggest that MRCKα might be a prognostic marker for breast cancer, but probably of limited functional importance.