To determine how the activity of the PI3K–AKT–mTOR pathway changes in MSCs after contact with MM cells, we investigated the phosphorylation of ribosomal protein S6 as an indicator of mTOR activity (20) and a biomarker of stromal cell response to PI3K–AKT–mTOR inhibition (21). This evidence concerns the gene RPS6 and Miyoshi myopathy.