tokens: In marked contrast to the previously published human CD6 sequence , the mouse sequence predicts a long cytoplasmic tail that is not closely related to other proteins and possesses two proline - rich motifs containing the SH3 - domain binding consensus sequence , three protein kinase C phosphorylation site motifs , nine casein kinase - 2 phosphorylation site motifs , and a serine - threonine - rich motif repeated three times .