What factors effect protein migration on SDS-PAGE

[KHUANG at CHEMVX.TAMU.EDU]

Hi, Netters,
I remember there had some discussion about protein migration on SDS-PAGE, but 
I could not pick them out. I have a few expressed protein in hands, some show 
higher molecular weight than predicted (from amico acid), some show lower M.W.
It is said the hrdrophobic region in the protein effect protein migration on 
SDS-PAGE, what are other factors?
Thanks

Huang