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WASHINGTON (Reuters) - Frozen blood from stored samples can be used to make cells resembling stem cells, researchers said on Thursday -- opening a potential new and easier source for the valued cells.They used cells from blood to make induced pluripotent stem cells or iPS cells -- lab-made cells that closely resemble human embryonic stem cells but are made from ordinary tissue.These iPS cells have in the past been made from plugs of skin, but blood is much easier to take from people and to store, the researchers reported in the journal Cell Stem Cell."Blood is the easiest, most accessible source of cells, because you'd rather have 20 milliliters of blood drawn than have a punch biopsy taken to get skin cells," Judith Staerk of the Whitehead Institute for Biomedical Research in Massachusetts, who worked on the study, said in a statement.Stem cells are the body's master cells, the source of renewed blood and tissue. So-called adult stem cells exist through life but they are partially developed.Embryonic stem cells from days-old embryos have the ability to become all the cell types in the body and also can proliferate in the lab for years.iPS cells are made by activating three or four genes that distinguish embryonic stem cells.Whitehead's Rudolf Jaenisch, who directed the work, said being able to use blood will open opportunities for researchers who want to use iPS cells to study how diseases develop."There are enormous resources -- blood banks with samples from patients that may hold the only viable cells from patients who may not be alive any more, or from the early stage of their diseases," Jaenisch said."Using this method, we can now resurrect those cells as induced pluripotent stem cells. If the patient had a neurodegenerative disease, you can use the iPS cells to study that disease."