In this context, it was considered to perform skin and kidney biopsies. field, such as potential novel therapeutic targets and prognosis of patients with MGCS. Abstract Monoclonal gammopathy of undetermined significance (MGUS) is defined as the presence of a monoclonal protein (M-protein) produced Upamostat by a small amount of plasma cells. The majority of patients remain asymptomatic; however, a fraction of them develop clinical manifestations related to the monoclonal gammopathy despite not fulfilling criteria of multiple myeloma or other lymphoproliferative disorder. These patients constitute an emerging clinical issue coined as monoclonal gammopathy of clinical significance (MGCS). The mechanisms involved are poorly understood, and literature is scarce regarding management. The clinical spectrum involves Upamostat symptoms related to renal, neurologic, skin, ocular, or bleeding manifestations, requiring a multidisciplinary approach. Treatment strategies rely on the basis of symptomatic disease and the M-protein isotype. In this review, we focus on MGCS other than renal, as the latter was earliest recognized and better known. We review the literature and discuss management from diagnosis to treatment based on illustrative cases from daily practice. Keywords: MGCS, MGUS, skin, ocular, bleeding 1. Introduction Monoclonal gammopathy of undetermined significance (MGUS) is defined by the presence of a monoclonal protein (M-protein) produced by a small B-cell/plasma cell clone in persons without features of symptomatic disease related to malignant disorders, such as multiple myeloma (MM), Waldenstr?m macroglobulinemia (WM), AL amyloidosis, or other lymphoproliferative disorder [1,2]. Prevalence is around 3% among people older than 50 years, and it increases with age [3]. Upamostat Nearly 80% of MGUS cases are derived from a non-IgM isotype (IgG or IgA), with IgG the most frequently found in population-based studies [4]. In the absence of myeloma-related symptoms, non-IgM MGUS is characterized by an M-protein lower than 30 g/L and less than 10% of plasma cells in bone marrow. Similarly, light-chain MGUS is based on an increased concentration of the involved light chain rather than a heavy-chain immunoglobulin expression, causing an abnormal free light chain ratio [2]. In the absence of WM-related symptoms, IgM MGUS is defined by an M-protein lower than 30 g/L and less than 10% of lymphoplasmacytic cells in the bone marrow [5,6]. Risk of progression to a malignant disorder (MM, WM, AL amyloidosis, or other B-cell lymphoproliferative disorder) is estimated in 1% annually [4,7]. This shortens the Upamostat overall survival (OS) for those who have MGUS compared to the control-matched population [4]. Despite the above mentioned, the majority of these patients will not progress to overt MM or other lymphoproliferative neoplasm and die from unrelated disorders. Apart from the potential malignant evolution, in some instances, the presence of a small B-cell/plasma cell clone producing a monoclonal immunoglobulin may cause a wide variety of clinical manifestations, leading Upamostat TNFRSF11A to a significant comorbidity and need for treatment [8,9]. Given the high heterogeneity of organs involved and clinical manifestations, this spectrum is categorized by an emergent concept recently coined as monoclonal gammopathy of clinical significance (MGCS) [10]. In many instances, the mechanisms underlying MGCS are unclear. Given the diversity of clinical manifestations, they are divided by organ involvement to better understand the pathophysiology, diagnosis, and treatment. In this sense and because of the high comorbidity and relative frequency, the most affected and well-studied organ is the kidney. So far, kidney disease related to the presence of an M-protein was already categorized as monoclonal gammopathy of renal significance (MGRS) [11,12,13]. Other MGCS are those involving the eyes, skin, peripheral nerves, and coagulation system, among others. Clinical manifestations may overlap with other unrelated M-protein diseases, making them hard to diagnose and to make related treatment decisions [14]. As MGRS, the aim of this review is to discuss MGCS other than renal, more recently recognized, highlighting practical diagnostic aspects and treatment approach. 2. Pathophysiology of MGCS The mechanisms by which the presence of the M-protein results in clinical syndromes not related to progression to a plasma cell or lymphoproliferative neoplasms can be divided according to findings on histopathology. Among these, the M-protein deposition is a common feature of many MGCS. As suggested by Fermand et al., the abnormal biochemical conformation of the monoclonal immunoglobulin results in its deposition that can be classified whether they have organized or non-organized ultrastructural appearance [10]. For instance, crystalline deposits of monoclonal immunoglobulin in the cornea are the main cause of keratopathy [15]. In the case of type 1 cryoglobulinemia,.