Most moles are not cancerous. However, there is a small risk of evolving into cancer, especially those moles which are already present at birth.
The "danger signs" to look out for in a mole are its:
A... asymmetry (whether it looks unequal when we draw an imaginary line to bisect the mole)
B... border (whether it is irregular and fuzzy)
C... colour (whether it has various shades, or changing in colour)
D... diameter (more than 6mm)
E... enlarging (out of proportion to the progress of the other moles in the body)
F... feels painful, or ulcerates, or bleeds, or simply "different" from the other moles in the body
