seemingly two bands of one band in SDS-PAGE?

[Soon Paik]

Kim wrote,
>Separation of protein bands of higher M.W ( 60 kDa) is
>ok, however that of lower one (25 kDa) is strange sometimes
>in that it has two forms of bands rather than one in
>SDS-PAGE. One band is slightly lower than the other. To
>my knowledge, proteolysis of the band (25 kDa) does not
>occur. Is this phenomenon artefact? How can I prevent it?

Is it possible that 25 kDa is present in both phosphorylated and 
dephosphoryalted forms, in which case the lower band is phosphoryalted 
form.

Soon Paik
Lombardi Cancer Center