unit of radiation absorption in the International System of Units (SI). The sievert has been recommended by the International Commission on Radiation Units and Measurements (ICRU) as a substitute for the rem, the long-standing special unit for measuring biological absorption of radiation. Like the rem, the sievert takes into account the relative biologic effectiveness (RBE) of ionizing radiation, since each form of such radiation—e.g., X rays, gamma rays, neutrons—has a slightly different effect on living tissue. Accordingly, one sievert is generally defined as the amount of radiation roughly equivalent in biologic effectiveness to one gray (or 100 rads) of gamma radiation. The sievert is inconveniently large for various applications, and so the millisievert (mSv), which equals ¹/_1,000 sievert, is frequently used instead. One millisievert corresponds to 10 ergs (erg) of energy of gamma radiation transferred to one gram of living tissue.