The missing number is 11.
The numbers in the sequence are all prime numbers, except for the missing number. The missing number must be a prime number that is less than 17, since the numbers in the sequence are all less than 17. The only prime number that is less than 17 and is not in the sequence is 11. Therefore, the missing number is 11.
Here is a table of the prime numbers less than 17:
Number | Prime? ------- | -------- 2 | Yes 3 | Yes 5 | Yes 7 | Yes 11 | Yes 13 | Yes 17 | Yes
As you can see, the only prime number that is less than 17 and is not in the sequence is 11. Therefore, the missing number is 11.