In islets from donors with T1D, islet hormone contents were reduced (Extended Data Fig. 10a–c), glucagon secretion was not increased when glucose was lowered from 6 mM to 1 mM (7 ± 9%) but stimulated by amino acids (+68 ± 2%, P = 0.00015) or CYN154806 (+68 ± 15%, P = 0.015; Fig. 4d). Here, GCG is linked to type 1 diabetes mellitus.