What are RIKEN genes?
My understanding is that these are cDNA sequences from the RIKEN project that don't yet have another gene name. The project carried out a comprehensive sequencing of the mouse genome, and as a result found a lot of cDNA transcripts that were previously unknown. Now they have some annotation (which can be varying degrees of functional annotation), and an accepted official identifier to allow people to talk about these transcripts when discovered in other studies.

It'd be interesting to see if any novel transcripts have been found from the Tabula Muris project. They would also need their own identifier, possibly leading to similar intrigue about their history.