In human genetics, Haplogroup U is a human mitochondrial DNA (mtDNA) haplogroup, a group of people who descend from a woman in the Haplogroup R (mtDNA) branch of the Genographic tree, who lived around 55,000 years ago. Mitochondrial DNA is passed from mother to daughter; it does not include the cell's nucleus where DNA is found, but still can be traced through maternal lines. Haplogroup U is found throughout Europe, and contains many subgroups, each reflecting unique geography and history. Among its subgroups is the Haplogroup K (mtDNA) branch. Known haplogroups are assigned the following letter codes: A, B, C, CZ, D, E, F, G, H, pre-HV, HV, I, J, pre-JT, JT, K, L0, L1, L2, L3, L4, L5, L6, L7, M, N, P, Q, R, S, T, U, UK, V, W, X, Y, and Z. If you are a Caucasian with haplogroup U, be aware that there is increased risk from prostate and renal cancer. The oldest haplogroup is L, which is found in sub-Saharan Africa.