IntroductionP21-activated kinase 1 (PAK1)is known to be overexpressed in several human tumour types, including breast cancer (BC). It is located on chromosome 11 (11q13.5-q14.1) and plays a significant role in proliferation in BC. In this study we aimed to assessPAK1gene copy number (CN) in primary breast tumours and their corresponding lymph node metastases, and associations betweenPAK1CN and proliferation status, molecular subtype, and prognosis. In addition, we aimed to study associations between CNs ofPAK1andCCND1. Both genes are located on the long arm of chromosome 11 (11q13).MethodsFluorescencein situhybridization forPAK1and Chromosome enumeration probe (CEP)11 were used on tissue microarray sections from a series of 512 BC cases. Copy numbers were estimated by counting the number of fluorescent signals forPAK1and CEP11 in 20 tumour cell nuclei. Pearson’s x2test was performed to assess associations betweenPAK1CN and tumour features, and betweenPAK1andCCND1CNs. Cumulative risk of death from BC and hazard ratios were estimated in analysis of prognosis.ResultsWe found meanPAK1CN ≥4<6 in 26 (5.1%) tumours, and CN ≥ 6 in 22 (4.3%) tumours. The proportion of cases with copy number increase (mean CN ≥4) was highest among HER2 type and Luminal B (HER2-) tumours. We found an association betweenPAK1CN increase, and high proliferation, and high histological grade, but not prognosis. Of cases withPAK1CN ≥ 6, 30% also hadCCND1CN ≥ 6.ConclusionsPAK1copy number increase is associated with high proliferation and high histological grade, but not with prognosis.PAK1CN increase was most frequent in the HER2 type and Luminal B (HER2-) subtype.PAK1CN increase is associated with CN increase ofCCND1.