Musashi is a very famous samurai in Japanese culture, but the term has a disparate meaning in biology. In 1994, Craig Montell, a neurobiologist, identified a gene that is crucial for the development of sensory organs in drosophila. Under the regulation of this gene, the sensory organ precursor (SOP) of wild-type drosophila divided asymmetrically into two different types of progenitors, i.e., neural and non-neural progenitors. Had this gene mutated, the SOP would split into two non-neural precursor cells, creating a symmetrical division. The formation of these two non-neural progenitor cells leads to a double-bristle phenotype in drosophila. Thus, this gene was termed Musashi because the double-bristle phenotype evokes the two swords of the Japanese samurai [1].

There are two highly conserved proteins in the Musashi family, i.e., Musashi1 (MSI1) and Musashi2 (MSI2) [2], [3]. In mammals, MSI1 contains 362 amino acids and has two RNA-recognizing motifs (RRM1 and RRM2) at the N-terminal that mediate the binding of MSI to target RNAs. Consisting of 328 amino acids, MSI2 also has two RRMs. The RRM1 and RRM2 of MSI1 consist of 20–110 and 109–186 amino acid sequences respectively. The RRM1 and RRM2 of MSI2 consist of 21–111 and 110–187 amino acid sequences respectively [4]. The homology of MSI1 and MSI2 in the amino acid sequence is ∼ 75 %. The similarity between the RRM1s and the RRM2s in MSIs is 78 % and 91 %, respectively [3]. The simplified structures of MSIs are shown in Fig. 1. However, the expression of MSI1 and MSI2 is highly tissue specific. MSI1 is mainly expressed in the nervous system, especially in the neural stem cells [5], [6], while MSI2 plays an indispensable role in the hematopoietic system, and is associated with many hematopoietic malignancies [7], [8], [9].

In addition to the differences in expression, there are also slight differences in the structural domains of MSI1 and MSI2. Two major protein-interacting domains are identified at the C-terminal of MSI1, one for the polyA tail-binding protein (PABP) [10] and the other for LIN28 [11]. No such domains, however, have been found in MSI2. PABP is a highly conserved protein that binds to polyA at the 3′-end of mRNA. It can participate in mRNA translation and regulate its stability by binding to PolyA. PABP also plays an important role in the regulation of early embryonic development and stem cell differentiation. PABP proteins with different structures and functions are continuously found in different cells. LIN28 is a vital regulator for miRNA biogenesis, and it can maintain stem cell stemness by inhibiting the activities of miRNA during neural differentiation of stem cells. MSI1 can enhance the nuclear localization of LIN28, inhibit the nuclear shearing step of miRNA, and eventually regulate the pluripotency of stem cells [12].