Because both ‐SIM‐cleaved unmethylated prelamin A (i.e., in Icmt deficiency) and non‐SIM‐cleaved unmethylated prelamin A (i.e., in Rce1 deficiency) appear to be less toxic than methylated prelamin A, these results suggest that the methyl group contributes to prelamin A’s toxic effect. The gene discussed is ICMT; the disease is hyperinsulinemic hypoglycemia, familial, 4.