Determining if a sequence is a valid DNA sequence

I'm attempting to write this program that reads in a sequence into a string variable, called sequence, and finds out if sequence contains a valid DNA sequence or not. I want to use a single for and one if-elif-elsestatement to determine whether the sequence is valid DNA or not.

This is what I have written so far:

sequence = input().upper()
dont_include = "BDEFHIJKLMNOPQRSUVWXYZ"
include = "ACGT"
sequence= input("Please enter a sequence:  ")
dont_include = dont_include.split(' ')
#sequence = sequence.split(' ')

common=0
for eachletter in sequence:
    if eachletter in dont_include:
        common +=1
    print(common)

elif eachletter in dont_include:
    print("This is not a valid DNA sequence")

else:
    print()

My program doesn't do exactly what I want... How can I improve it?