  Over the last few years, Sr$_2$IrO$_4$, a single-layer member of the
Ruddlesden-Popper series iridates, has received much attention as a close
analog of cuprate high-temperature superconductors. Although there is not yet
firm evidence for superconductivity, a remarkable range of cuprate
phenomenology has been reproduced in electron- and hole-doped iridates
including pseudogaps, Fermi arcs, and $d$-wave gaps. Further, a number of
symmetry breaking orders reminiscent of those decorating the cuprate phase
diagram have been reported using various experimental probes. We discuss how
the electronic structures of Sr$_2$IrO$_4$ through strong spin-orbit coupling
leads to the low-energy physics that had long been unique to cuprates, what the
similarities and differences between cuprates and iridates are, and how these
advance the field of high-temperature superconductivity by isolating essential
ingredients of superconductivity from a rich array of phenomena that surround
it. Finally, we comment on the prospect of finding a new high-temperature
superconductor based on the iridate series.
