An odd prime p is called a cluster prime if every even positive integer less than p - 2 can be written as a difference of two primes q - q', where q, q'<=p. The first 23 odd primes 3, 5, 7, ..., 89 are all cluster primes. The first few odd primes that are not cluster primes are 97, 127, 149, 191, 211, ... (OEIS A038133). The numbers of cluster primes less than 10^1, 10^2, ... are 23, 99, 420, 1807, ... (OEIS A039506), and the corresponding numbers of noncluster primes are 0, 1, 68, 808, 7784, ... (OEIS A039507).