CD44 and cancer: Moreover, Yang et al., comparing the expression of cluster of differentiation 44 (CD44), a well-studied tumour marker associated with gastric cancer stem cells, with RKIP expression under different pathological conditions, demonstrated that RKIP exhibited a negative effect on initial tumour development, and that the downregulation of RKIP in the advanced stages of cancer facilitated CD44 and peroxiredoxin 2 overexpression.