Nanogram (ng) A unit of weight consisting of one billionth  of a gram (1 x 10-9 g). Necrosis Death, usually referring to cell death. NEL No Effect Level.  A quantity of a substance that is below the threshold on the dose-response curve. Neonates Newborn animals. Neoplasm An uncontrolled and progressive growth of cells.  It may be benign or malignant.  same as Tumor. Neoplastic Pertaining to or like a neoplasm or neoplasia (tumor). Nephrotoxin A systemic poison whose target is the kidney. Neurotoxicity Toxicity to cells of the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves outside the CNS). Neurotoxin A systemic poison whose target organ is the nervous system. New Drug Application (NDA) The process by which a manufacturer of a new drug applies to the Food and Drug Administration for formal approval to market the drug. NIOSH National Institute of Occupational Safety and Health.  It is an institute in the Public Health Service that conducts research on health hazards in the workplace. NOAEL No Observed Adverse Effect Level.  The highest dose in a toxicity study at which there were no toxic or adverse effects observed. Non-Clinical Laboratory Study A study of a pharmaceutical performed with laboratory animals which provides the basis for human clinical investigations.