Rand, a research think tank, estimated there are between 1,320 and 6,630 transgender Americans serving in the military out of 1.3 million active-service members. The Williams Institute, a think tank housed at UCLA’s law school, puts the total number of active-duty and guard or reserve service members higher, estimating that 15,500 transgender people are part of those military forces. The institute’s researchers also calculated that 134,300 veterans identify as transgender.

Although it’s impossible to know how many members would need gender-transition-related services, it’s likely that only a fraction would want to transition while in service—Rand suggested that somewhere between 29 and 129 people per year would make those requests. The “upper bound” of estimated requests was “0.1 percent of the total force,” Rand wrote. The expected costs related to these services were between $2.4 million and $8.4 million each year.

When Mattis announced the delay in implementing the Obama-era guidelines, he questioned how the policy would affect service members’ ability to perform their duties. “Since becoming the Secretary of Defense, I have emphasized that the Department of Defense must measure each policy decision against one critical standard: Will the decision affect the readiness and lethality of the force?” Mattis said, according to The Washington Post. “Put another way, how will the decision affect the ability of America’s military to defend the nation?”

While there has not been much research done on this question, according to Rand, policies welcoming lesbian, gay, and bisexual Americans to serve openly in the military did not diminish units’ effectiveness. The Rand researchers expected “little or no impact on unit cohesion, operational effectiveness, or readiness” resulting from Obama’s policy.

Former Secretary Carter issued a statement criticizing Trump’s decision on Wednesday morning. “I continue to maintain that what matters in choosing those who serve is that they are best qualified. To choose service members on other grounds that military qualifications is social policy and has no place in our military,” he said. “There are already transgender individuals who are serving capably and honorably. This action would also send the wrong signal to a younger generation thinking about military service.”

Legislators have also been working on potential ways to reverse or gut funding for the Obama administration’s policy. Foreign Policy reported on Tuesday that Vice President Mike Pence had been working with Republican Congressmen on amendments to the 2018 defense-spending bill that would have prohibited money from being used for medical services related to gender transition.This was a policy that many Republicans hated: During Congressional debates over the issue this month, California Representative Duncan Hunter, a veteran of the wars in Iraq and Afghanistan, said of transgender people, “You’re joining the U.S. military. Choose what gender you are before you join.”