Let's analyze the pattern:
- First, observe the positions of the letters in the alphabet:
- R = 18th letter
- U = 21st letter
- X = 24th letter
- A = 1st letter
- D = 4th letter
The sequence of positions is: 18, 21, 24, 1, (4)
The difference between the positions is:
- 21 - 18 = 3
- 24 - 21 = 3
- 1 - 24 = -23
- 4 - 1 = 3
So, the pattern follows alternating steps of +3, with one large jump (-23) followed by another +(3)
Thus, the next letter would be at the 7th position in the alphabet, which corresponds to the letter G.
Therefore, the missing letter is \( \boxed{G} \).