See-through medaka lines are suitable for observing internal organs throughout life. They were bred by crossing multiple color mutants. However, some of the causal genes for these mutants have not been identified. The medaka has four pigment cell types: black melanophores, yellow xanthophores, white leucophores, and silvery iridophores. The causal genes of melanophore, xanthophore, and leucophore mutants have been elucidated, but the causal gene for the iridophore mutant remains unknown. Here, we describe the iridophore mutant, guanineless (gu), which exhibits a strong reduction in visible iridophores throughout its larval to adult stages. The gu locus was previously mapped to chromosome 5, but was located near the telomeric region, making it difficult to integrate into the chromosome. We sought the causal gene of gu using synteny analysis with the zebrafish genome and found a strong candidate, purine nucleoside phosphorylase 4a (pnp4a). Gene targeting and complementation testing showed that pnp4a is the causal gene of gu. This result will allow the establishment of inbred medaka strains or other useful strains with see-through phenotypes without major disruption in the genetic background of each strain.KEYWORDS medaka iridophore pnp4a pigment mutant guanineless Medaka embryos are transparent and develop externally, making them suitable for the observation of internal organ developmental processes. However, because of the gradual appearance of pigment cells, observation of internal organs from the early juvenile to adult stages becomes difficult. To solve this problem, the see-through strains ST-II, ST-V, and SK2 were bred by crossing multiple medaka color mutants (Wakamatsu et al. 2001;Ohshima et al. 2013). In zebrafish, the casper fish were made for this purpose. The casper fish are doubly mutant for nacre (microphthalmia-associated transcription factor a mutant) and roy orbison. They show a complete lack of melanophores and iridophores and have an almost transparent body from the early juvenile to adult stage (White et al.