**4. Conclusion**

The need to identify biomarkers for complex systemic and chronic diseases is pressing, with an increasing push towards the successful development of therapies aimed at modulating serum levels, blocking receptor binding or inhibiting signalling cascades. HSPs hold great potential as therapeutic targets for those conditions with underlying mechanisms involving accumulation of misfolded or damaged proteins, oxidative stress, altered mitochondrial bioenergetics or dysregulated apoptosis, particularly as a result of their non-chaperoning functions. Studies presented herein suggest that circulating HSP levels may be exploited as biomarkers of such conditions, with cancer and cardiovascular complications linked to T2DM being the contexts used to exemplify.

A major limitation of most studies performed on extracellular HSPs is that their functions and roles in disease have not been elucidated yet. As a result the biochemistry and signalling are investigated very poorly, such as testing for a single downstream product of a complex cascade which can be affected by multiple inputs. Similarly, the PTMs on extracellular HSPs are still in their majority obscure both in abundance and functional significance. Studies conducted retrospectively, on single HSPs in isolation, using small patient groups and without adjustment for confounding effects offer a very poor analysis of the predictive power of HSPs for early diagnosis or prognostic assessment. Thus, in future research, it is important to take into consideration that HSPs do not work in isolation, but act within a network, rather than just detect changes in the total extracellular expression levels of individual HSPs and analyse changes in both total HSP and specific PTMs within groups of chaperone proteins that are functionally relevant to either the development of or resultant from the progression of the condition under investigation. Furthermore, this needs to be performed in large cohorts of well-characterised patients, with prospective validation of promising biomarker panels, if the intent is really their application in a clinical setting.
