Bone marrow is a soft fatty material, found in the center of large bones, that produces new blood cells for the body. Bone marrow was traditionally donated by extracting the marrow from donors’ bones with a long needle (i.e., aspiration). However, in the last 20 years, a new technique called peripheral blood stem cell apheresis was introduced. Through peripheral blood stem cell apheresis, the necessary stem cells are extracted by taking blood from donors’ veins. The National Organ Transplant Act (Act) classified bone marrow as a human organ and prohibited individuals from being compensated for their bone-marrow donations, thus limiting the number of potential donors. Parents of children with terminal diseases, a physician, and MoreMarrowDonors.org, a not-for-profit advocacy organization (plaintiffs), filed suit in federal district court against Eric Holder, Jr., (defendant) in his official capacity as Attorney General of the United States. Plaintiffs claimed that the Act violated the Equal Protection Clause of the U.S. Constitution because the Act prohibited compensation for donating bone marrow but did not prohibit compensation for donating blood, sperm, or eggs. According to plaintiffs, bone-marrow collection through apheresis could be accomplished quickly and without removing marrow, and the donor's body would quickly regenerate the donated stem cells. Thus, plaintiffs claimed the distinction between bone-marrow donation and the other types of compensable donation had no rational basis and was an equal-protection violation. The district court dismissed plaintiffs’ complaint for failure to state a claim upon which relief could be granted. Plaintiffs appealed.