In the small intestine, stem cells are considered to exist at the bottom of the crypt. Actively proliferatingtransitional cells supplied from stem cells are differentiated into two directions upward and downward. Upwardcells are differentiated into absorbing epithelial cells, goblet cells, and endocrine cells, and downwardcells differentiated into Paneth cells. However there are some difficulties to identify the stem cells becauseof their unique characteristics. At first, stem cells occur as actual stem cells and potential stem cells, andsecond, there is diversity in stem cells. Therefore, molecules suitable for a marker of small intestinal stemcells are necessary to distinguish "true" stem cells from others. Energetically searched for in recent years,Musashi-1, type 1A bone morphogenetic protein receptor (BMPR-1A), phospho-phosphatase and tensinhomolog deleted on chromosome ten( phospho-PTEN), doublecortin and calmodulin kinase-like-1( DCAMKL1),ephrin receptors( Eph receptors), integrins, and leucine-rich repeat-containing G protein-coupled receptor5 (Lgr5) are proposed. Among them, Musashi-1 draws attention as one of a candidate marker forsmall intestinal stem cells. We here introduce our reviews about expression of Musashi-1 and Hes1 proteinsin the small intestine, and would like to overview the way to identify the small intestinal stem cells