ic ient ▁in ▁these ▁en zym es ▁( H G PR r - ▁and / or ▁T K -) ▁can ▁not ▁divide ▁( since ▁they ▁cannot ▁produce ▁pur ines ▁and ▁p yr im id ines ▁due ▁to ▁the ▁am in opter in ▁present ▁in ▁the ▁H AT ▁medium ). ▁ ▁For ▁using ▁H AT ▁medium ▁as ▁a ▁select ive ▁agent , ▁human ▁cells ▁used ▁for ▁fusion ▁must ▁be ▁def ic ient ▁for ▁either ▁the ▁en zym e ▁H G PR T ▁or ▁T K , ▁while ▁mouse ▁cells ▁must ▁be ▁def ic ient ▁for ▁the ▁other ▁en zym e ▁of ▁this ▁pair . ▁Thus , ▁one ▁may ▁f use ▁H G PR T ▁def ic ient ▁human ▁cells ▁( design ated ▁as ▁T K + ▁H G PR r -) ▁with ▁T K ▁def ic ient ▁mouse ▁cells ▁( den oted ▁as ▁T K - ▁H G PR T + ). ▁ ▁Their ▁fusion ▁products ▁( hy brid ▁cells ) ▁will ▁be ▁T K + ▁( due ▁to ▁the ▁human ▁gene ) ▁and ▁H G PR T + ▁( due ▁to ▁the ▁mouse ▁gene ) ▁and ▁will ▁multiply ▁on ▁the ▁H AT ▁medium , ▁while ▁the ▁man ▁and ▁mouse ▁cells ▁will ▁fail ▁to ▁do ▁so . ▁Exper iments ▁with ▁other ▁select ive ▁media ▁can ▁be ▁planned ▁in ▁a ▁similar ▁fashion . ▁ ▁Character istics ▁of ▁som atic ▁hy brid ization ▁and ▁cy brid ization ▁▁▁ ▁Som atic ▁cell ▁fusion ▁appears ▁to ▁be ▁the ▁only ▁means ▁through ▁which ▁two ▁different ▁parent al ▁genom es