Allow me to be the acronym buster (self-proclaimed 🙂
MPN = Myeloproliferative neoplasms, a group of diseases that affect blood-cell formation.
ET = Essential thrombocythemia, an uncommon disorder in which your body produces too many platelets.
PV = polycythemia vera, a blood disorder in which the body makes too many red blood cells.
MF = Myelofibrosis, an uncommon type of bone marrow cancer that disrupts your body's normal production of blood cells.
(ST, I believe this was a typo for ET.)
The JAK2 mutation is acquired and not inherited. People are not born with this mutation, but instead develop it in their bone marrow cells sometime later in life. @pattitoo, I would question your doctor about this and encourage you to get clearer answers.