DNA-based storage is considered to be a promising option to accommodate huge amounts of data. The strings of nucleotides are prone to errors though. To reduce the error probability, these strings should satisfy constraints on the ratio of A's and T's versus the number of G's and C's, and on the maximum number of repeated identical nucleotides. To deal with errors when they occur after all, it is also desirable that the set of DNA-strings possesses certain error correction or detection capabilities. This is established by designing quaternary constrained codes with a specified minimum distance. Here, maximum-sized block codes with a fixed number of G/C symbols, no symbol repetition, and a minimum Hamming distance of two are presented.