Fifty-two U.S. House members have signed a letter voicing concerns about a proposal to change the map that governs how donated livers are distributed around the country.

The letter, dated Friday, was written by Kansas Congressman Kevin Yoder, a Republican. It comes in advance of a meeting Tuesday in Chicago to discuss a proposal to redraw the nation’s transplant regions.

The issue is that some areas have fewer donated organs, and higher demand for them, than others. The United Network for Organ Sharing, which runs the nation’s transplant network, has proposed basing the map on the distribution and demand for donated organs.

The lawmakers who signed the letter are largely from the South and the Midwest. Those regions have higher organ donation rates and fear they would be negatively affected.