All individuals showed markedly reduced insulin secretion in utero, as they all had low birth weight (median −2.98 s.d., interquartile range (IQR) −3.56 to −2.47) reflecting reduced insulin-mediated fetal growth and also postdelivery, as they rapidly developed insulin-requiring diabetes (median age at diagnosis 17 weeks, IQR 3–23). The gene discussed is INS; the disease is diabetes mellitus.