Ameren Corporation (Ameren) is a utility holding company. The Company�s principal subsidiaries are Union Electric Company (Ameren Missouri) and Ameren Illinois Company (Ameren Illinois). The Company's segments include Ameren Missouri and Ameren Illinois. Ameren Missouri operates a rate-regulated electric generation, transmission and distribution business, and a rate-regulated natural gas transmission and distribution business in Missouri. Ameren Illinois operates a rate-regulated electric and natural gas transmission and distribution business in Illinois. AER consists of non-rate-regulated operations, including Ameren Energy Generating Company (Genco), Ameren Energy Resources Generating Company (AERG), Ameren Energy Resources Company and Ameren Energy Marketing Company (Marketing Company). Ameren�s portfolio of natural gas supply resources includes firm transportation capacity and firm no-notice storage capacity leased from interstate pipelines.