As the cDNA sequences of green algal sulP were used in the multiple alignments, there was no usable conserved region to design either specific primer or degenerate primer. Thus, the strategy was changed to do the protein alignment. Five protein primary sequences of algal sulP were retrieved from NCBI database: Nephroselmis olivacea (Acc No. NP_050928), Chlorella vulgaris (Acc No. NP_045890), Chlamydomonas reinhardtii (Acc No. XP_001692459), Mesostigma viride (Acc No. NP_038441), Chlorokybus atmophyticus (Acc No. YP_001019170). The alignment showed the conserved regions on the middle and the C-terminus. The cDNA sequences correspondinding to the conserved parts were used as templates in degenerate primer design. More than a thousand combinations of degenerate forward primers (SdegF), two combinations of nested forward primers (SnestedF) and over a hundred combinations of degenerate reverse primers (SdegR) were designed.