Is there a difference between ISH and ISHH? (In Situ Hybridization, In Situ Hybridization Histochemistry)
The term appears to be in use as a synonym.  PLOS ONE is a good journal, and in it Okubo, 2016 write &quot;The protocol for ISHH was described in detail previously [22].&quot;  Where [22] is Yamanaka, 2007, describing a classic ISH protocol.
Note that both papers use good old fashioned nuclear tract emulsion for S-35 labelled probes.  The ISHH protocol is in its own section separate from IHC.  It seems a bit odd because no literal &quot;chemistry&quot; is involved in recognizing the target - the RNA probe binds non-covalently and emits radiation.  However, as noted in the question discussion, IHC is formally defined (OED) as &quot;the branch of science concerned with the identification and distribution of the chemical constituents of tissues by means of stains, indicators, and microscopy&quot;.  For example, though I have seen immunofluorescence and immunohistochemistry used to describe mutually exclusive visualization options, noncovalent binding of fluorophore-labelled antibodies is described as &quot;fluorescent IHC&quot;.