This study investigates the relationship between vitamin D deficiency and uncontrolled type 2 diabetes mellitus (T2DM) indicated by elevated glycosylated hemoglobin (HbA1c) levels, alongside assessing the association between fasting C peptide levels and uncontrolled T2DM, considering their roles in β-cell function and insulin secretion. The gene discussed is INS; the disease is type 2 diabetes mellitus.