. ▁This ▁medium ▁is ▁supp lement ed ▁with ▁hy po x anth ine , ▁am in opter in ▁and ▁th ym id ine , ▁hence ▁the ▁name ▁H AT ▁medium . ▁Ant imet abol ite ▁am in opter in ▁blocks ▁the ▁cell ular ▁b ios yn thesis ▁of ▁pur ines ▁and ▁p yr im id ines ▁from ▁simple ▁sug ars ▁and ▁am ino ▁ac ids . ▁ ▁However , ▁normal ▁human ▁and ▁mouse ▁cells ▁can ▁still ▁multiply ▁as ▁they ▁can ▁util ize ▁hy po x anth ine ▁and ▁th ym id ine ▁present ▁in ▁the ▁medium ▁through ▁a ▁salv age ▁path way , ▁which ▁ordin arily ▁rec y cles ▁the ▁pur ines ▁and ▁p yr im id ines ▁produced ▁from ▁de grad ation ▁of ▁nucle ic ▁ac ids . ▁ ▁Hy po x anth ine ▁is ▁converted ▁into ▁gu an ine ▁by ▁the ▁en zym e ▁hy po x anth ine - gu an ine ▁ph osph or ib os yl trans fer ase ▁( H G PR T ), ▁while ▁th ym id ine ▁is ▁ph osph ory l ated ▁by ▁th ym id ine ▁kin ase ▁( T K ); ▁both ▁H G PR T ▁and ▁T K ▁are ▁en zym es ▁of ▁the ▁salv age ▁path way . ▁ ▁On ▁a ▁H AT ▁medium , ▁only ▁those ▁cells ▁that ▁have ▁active ▁H G PR T ▁( H G PR T +) ▁and ▁T K ▁( T K +) ▁en zym es ▁can ▁pro l ifer ate , ▁while ▁those ▁def