Cyclin D1 expression in MM PCs with and without RAS mutations. The actual cyclin D1-β-actin ratio was determined in FACS-purified MM PCs with and without RAS mutations using real-time PCR as described in “Patients, materials, and methods.” PCs with high (cyclin D1-β-actin ratio > 0.1) and low to intermediate (cyclin D1-β-actin ratio < 0.1) cyclin D1 expression were identified.