34443...34443 (25-digits)

This number is a prime.

                                                 34443 4343444344 4343434443

The only X-prime of order 5. (An X-prime is a palindromic prime of n^2 digits (n, odd > 2) arranged left-right, top-down, in an n-by-n array, where the digits of the two diagonals are equal to a and all the other digits are equal to b different from a). [Loungrides]