Related Clues:

• Matchless, e.g.
• Unlike 666
• Like all but one prime
• Prompting a 'hmm,' perhaps
• Like all prime numbers except 2
• Like 1, 3, 5, 7 ...
• Really strange
• Like 9 or 5
• Hardly commonplace
• Like 1, 11, 111, and 1,111
• Like an unpaired sock
• Abnormal
• Hard to explain
• See 127 Across
• Not ordinary