mycophenolic acid,aminopterin, MAX

[may]

I am currently using mycophenolic acid, aminopterin and xanthine (MAX) selectio
n to maintain my transformed fibroblast clones, but when I grow them in non-sel
ective medium (no mycophenolic acid, aminopterin, but with xanthine and other b
ases) for several days and switch back to MAX medium, the cells start dying.  D
oes anyone has experience working with MAX selection?  Please help! this result
s drive me crazy!!!!!!!!