We developed mouse model systems to investigate the potential for cyclin D1 to induce CIN in vivo. In a mammary gland specific Tet-inducible model the acute expression profile regulated by cyclin D1 after 7 days was enriched in genes that rank highly with CIN. We also used a mammary gland targeted model (MMTV) to continuously express cyclin D1. The mice started to develop mammary gland tumors at 400 days and the tumor-free incidence was 40% in MMTV-cyclin D1. The gene expression profile of the tumors showed enrichment for the CIN signature. We next compared cyclin D1 expression and the highest ranking CIN genes to a breast cancer expression database and discovered that expression of genes promoting CIN are highly enriched in luminal subtype and that high cyclin D1 and CIN expression correlate specifically in the luminal B subtype. There is increasing interest in employing drugs in the clinic that exploit CIN in tumors. The high CIN expression index in luminal B breast cancer provides a basis for using Cdk and CIN inhibitors as a targeted therapeutic approach.