As written...H_3stackrel(1)C-CHBrCHBrstackrel(4)CH_3 (I had trouble representing this on the editor...the chiral carbons, stackrel2C and stackrel3C are not labelled but their substitution is evident).
Anyway, the beast has two chiral centres... In principle it has 2^n isomers where n=2, i.e. n="the number of chiral carbons"; POTENTIALLY 4 diastereomers....
Now "RR" is the mirror image of "SS"...and these are non-superposable, and I presume that only the ONE of these enantiomers is biochemically active....and the industrial synthesis would produce this isomer. But the "RS" disastereomer is THE SAME as the "SR" disastereomer.....given the symmetry of the molecule. With me? You will see this relationship if you make a model.