In one aspect, affinity tags for recombinant protein purification are described herein which, in some embodiments, can mitigate or overcome disadvantages of prior affinity tag systems. In some embodiments, for example, affinity tags described herein permit efficient elution of desired recombinant proteins with simplified solution systems, such as alkali metal salt solutions. An affinity tag described herein comprises an amino acid sequence including a repeating amino acid unit of BXXXBXX, wherein B is an amino acid selected from the group consisting of histidine, lysine and arginine and X is an amino acid selected from the group consisting of amino acids other than histidine, lysine and arginine.