Thousands of potential organ donors are not on Texas's official donor registry -- even though they think they are.
Organ donor groups say as many as seven out of 10 people who think they are donors actually aren't.
Potential donors should check to see if they are listed on the donor registry, the Southwest Transplant Alliance says. (Click here to check your registration.)
Southwest Transplant Alliance spokeswoman Pam Silvestri said that no one should assume he or she is registered.
"It could be that you said yes at the driver's license office and a key didn't get punched, or it could be you thought you said yes and you didn't," she said.
Cynthia Jefferson, whose son needs a kidney transplant, thought she had signed up when she renewed her driver's license, but her name was not on the registry.
"I really was surprised, because I know that I checked the box on the button," she said.
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Silvestri said the registry, which was created in 2006, is fairly new.
Once someone registers, it serves as a legal and binding contract, unlike the "donor cards" many people carry.
"We printed millions of those things all across the country," she said. "They were so useless, because if you signed a donor card and got two witnesses, you tattooed it on your arm and you had a necklace that said, 'I want to be an organ donor,' it didn't matter because your family still had to make that decision. There was not a legal document in place."
Jefferson's son, Bryson, is hooked to an at-home dialysis machine for 10 hours every day in between schoolwork and band practice. More potential donors on the registry mean he has a greater chance of receiving a new kidney.
"I would have a little bit more freedom," he said. "I would be to do more stuff that I would like that I can't do now," he said.
After learning she wasn't on the Texas registry, his mother went online and reregistered.
"I am checked now for all of my organs to give life for somebody when the time comes," she said.
More: Donate Life Texas Registry
NBC DFW's Grant Stinchfield contributed to this report.