If a cell contains "Yes", another cell MUST be filled, else, another cell cannot be filled

[Kappany]

Hi All

I have in cell B2 a drop-down list with "Yes" or "No", I need to make a criteria to do the following:

If B2 = "Yes", then C2 & D2 & E2,.... MUST be filled (From a Drop-Down lists if needed too).
Edit

and if I tried to move from C2 to any other cell, I need an Error Message tells me that "You MUST fill in this cell"

End Edit

If B2 = "No", then C2, D2, E2,.... CANNOT be filled at all.

Thanks in advance.

[oeldere]

a2=if(a2="yes",c2&d2&e2,"")