CCND1 and breast cancer: The Integrative Cluster 2 represents a particularly high-risk subgroup of luminal B BC patients, characterised by two separate amplicons in chromosome 11: one at 11q13.3 (CCND1 amplification) and a separate peak from 11q13.5–11q14.1 centred around PAK1, RSF1, C11orf67 and INTS4. Although our data are obtained from a small number of PDX models, they suggest that this group of patients could potentially benefit from PLK1 targeting.