Health insurer Cigna Corp. plans to buy Express Scripts Holding Co. in a cash-and-stock deal worth $54 billion that the companies say will expand their health care offerings and help them control costs.

Under the terms of the agreement, Express Scripts shareholders will receive $48.75 in cash and 0.2434 shares in the combined company for each Express Scripts share. The consideration amounts to about $96.03 for each Express Scripts share, a 31% premium over Express Scripts’ Wednesday closing price of $73.42.

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