Protein kinase D (PKD) is a member of a class of serine/threonine protein kinases that modulate numerous biological activities, such as protein transport, cell migration, differentiation, proliferation, apoptosis, EMT and immune regulation.9–15 PKD3 expression is increased in highly invasive breast and prostate cancers.9–15 The concomitant absence of PKD1 in highly invasive breast cancer indicates that while this protein plays an anti-tumour role, PKD3 may instead promote cancer. The gene discussed is PRKD3; the disease is breast cancer.