Currently used markers of metabolic control, i.e. glucose levels, continuous glucose measurements, HbA1c etc., can be used to ensure the fulfilment of treatment criteria, but fail to reflect β-cell mass in both T1D and T2D since these markers are primarily affected by the treatment regime (i.e. exogenous insulin in T1D and insulin and/or oral antidiabetic drugs and GLP-1 analogs in T2D). The gene discussed is INS; the disease is type 1 diabetes mellitus.