Please see these articles covering how to use subscripted expressions as variables:
http://stackoverflow.com/questions/5481216/subscripted-variables
http://stackoverflow.com/questions/4253883/dealing-with-subscripted-variables
Subscript[M,5] is not a symbol, but an expression. This will make things confusing.
I very much recommend avoiding using Subscript or SuperScript or the like while writing code unless you are already familiar with what is outlined in the two articles above.
To answer your exact question, you want to use SubSuperScript:
http://reference.wolfram.com/mathematica/ref/Subsuperscript.html
