Essential thrombocythemia (ET) is one of the classical Philadelphia-negative myeloproliferative neoplasms with different mutations that can be associated with it, like Janus kinase 2 (JAK2), myeloproliferative leukemia protein (MPL), and Calreticulin (CALR) (types 1 and 2). However, there is a lack in the literature concerning other types of CALR mutations and their clinical significance and prognosis. Here we report a 42-year-old male with type 2 diabetes who presented with an inferior ST-elevation myocardial infarction and thrombocytosis. The diagnosis of ET with CALR (neither type 1 nor type 2) was confirmed, which suggests the pathognomonic feature of this mutation.