nitrogen mustard, any of various poisonous compounds originally developed for military use (see poison gas). Like mustard gas and lewisite, it is a vesicant (blistering agent). In the form of its crystalline hydrochloride it is used as a drug in the treatment of Hodgkin's disease, non-Hodgkin's lymphomas, and brain tumors. Nitrogen mustards cause mutations in the genetic material of cells, thereby disrupting mitosis, or cell division. Cells vary in their susceptibility to nitrogen mustards, with rapidly proliferating tumor and cancer cells most sensitive; bone marrow, which produces red blood cells, is also sensitive, and depression of red blood cell production is a frequent side effect of nitrogen mustard therapy. The nitrogen mustards also suppress the immune response (see immunity).

mustard gas, chemical compound used as a poison gas in World War I. The burning sensation it causes on contact with the skin is similar to that caused by oil from black mustard seeds. The compound is not a gas but a colorless, oily liquid with a somewhat sweet, agreeable odor; it boils at 217°C;. A powerful vesicant, mustard gas causes severe blistering even in small quantities. Highly irritating to the eyes, it quickly causes conjunctivitis and blindness. If inhaled, it attacks the respiratory tract and lungs, causing pulmonary edema. Some effects of exposure to mustard gas are delayed up to 12 hr; death may result several days after exposure. Mustard gas was introduced by the Germans in warfare against the British at Ypres, Belgium, in July, 1917, and took a heavy toll of casualties. It is dispersed as an aerosol by a bursting shell. Chemically, mustard gas is a thioether, 2,2'-dichlorodiethyl sulfide, (ClCH₂CH₂)₂S. It can be prepared by reacting ethylene with sulfur monochloride, S₂Cl₂, or by other methods. Its vesicant property is readily destroyed either by oxidation or by chlorination (e.g., with bleaching powder).

mustard, common name for the Cruciferae, a large family chiefly of herbs of north temperate regions. The easily distinguished flowers of the Cruciferae have four petals arranged diagonally ("cruciform") and alternating with the four sepals. Most of the nearly 50 genera indigenous to the United States are found in the West. The family includes numerous weeds and wildflowers, e.g., peppergrass, toothwort, and shepherd's-purse. The Cruciferae, often rich in sulfur compounds and in vitamin C, include important food and condiment plants, many cultivated from ancient times. Especially important are the herbs of the genus Brassica, e.g., rape, rutabaga, turnip, mustard, and numerous varieties of the cabbage species. Cress, watercress, horse-radish, and radish are also of this family. A few species are cultivated as ornamentals, e.g., candytuft, rose of Jericho, wallflower, and types of stock, rocket, and alyssum. Woad was formerly an important dye source. The herbs of the family that are called mustard are species of Brassica native to Europe and W Asia. Most important commercially are the black mustard (B. nigra) and white mustard (B. alba). These are yellow-flowered annuals naturalized in the United States; the black mustard is often a weed infesting grainfields, as is also the charlock, or wild mustard (B. arvensis). The black and the white mustard resemble each other and are used more or less similarly. They are cultivated for the seeds, which are ground and used as a condiment, usually mixed to a paste with vinegar or oil, sometimes with spices or with an admixture of starch to reduce the pungency. (The pungency of mustard does not develop until it is moistened.) Mustards are also grown as salad plants and for greens, as are the Indian, or leaf, mustard (B. juncea) and the Chinese mustard, or bok-choi (B. chinensis). The white mustard is used in some places as forage for sheep and as green manure. Black mustard seeds are more pungent than the white and yield a yellowish, biting oil (mustard oil) that has also been useful in medicine. Mustard is classified in the divison Magnoliophyta, class Magnoliopsida, order Capparales, family Cruciferae.

Family Brassicaceae (or Cruciferae), composed of 350 genera of mostly herbaceous plants with peppery-flavored leaves. The pungent seeds of some species lead the spice trade in volume traded. Mustard flowers take the form of a Greek cross, with four petals, usually white, yellow, or lavender, and an equal number of sepals. The seeds are produced in podlike fruits. Members of the mustard family include many plants of economic importance that have been extensively altered and domesticated by humans. The most important genus is Brassica (see brassica); turnips, radishes, rutabagas, and many ornamental plants are also members of the family. As a spice, mustard is sold in seed, powder, or paste form.

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