We propose that the following parameters can be used to define SMCL: lack of B symptoms, normal serum lactic dehydrogenase and β2-microglobulin levels; low MCL-International Prognostic Index (MIPI) score, maximum tumor diameter < 3 cm, spleen size <20 cm, Positron Emission Tomography/computerized tomography (PET/CT) with the Standard Uptake Value (SUV) max <6; Ki-67 < 30%; absence of c-myc, TP53 and NOTCH1/2 mutations, non-blastoid/pleomorphic histology; and no tumor growth on re-evaluation every 2~3 months (followed for at least 6 months). Here, TP53 is linked to neoplasm.