To determine if CD161-expressing cells are likely to predict early complications, namely, mucositis (≥grade 3), infections, and cytomegalovirus (CMV) reactivation, we prospectively examined CD161-expressing cells (CD3+CD4+CD161+ and CD3+CD8+CD161+) in peripheral blood samples from 108 patients with MM undergoing ASCT. This evidence concerns the gene KLRB1 and mucositis.