Immature human egg cells skip a fundamental metabolic reaction thought to be essential for generating energy. This helps cells avoid creating reactive oxygen species, harmful molecules that can accumulate, damage DNA and cause cell death. The findings explain how human egg cells remain dormant in ovaries for up to 50 years without losing their reproductive capacity. The findings could lead to new strategies to preserve the ovarian reserves of women undergoing cancer treatment, as well as open new lines of research that explain why one in four cases of infertility are unexplained.