Hypothetical model of mRNA sorting in megakaryocytes. The mechanism by which mRNAs are sorted in megakaryocytes is not known. This model is based on an analogy to the more well-characterized neuronal model. DNA is transcribed into mRNA in the nucleus of the megakaryocyte. Some transcripts (eg, MMP1) bind an mRNA binding protein, forming a complex that translocates to the cytoplasm. The complex associates with a kinesin motor, is transported through the proplatelet by microtubules, and is ultimately delivered to the nascent platelet. Some transcripts that are delivered to the platelet are transcribed constitutively. Others are transcribed in an activation-dependent manner. Transcripts that fail to bind an mRNA binding protein are translated into proteins in the rough endoplasmic reticulum (RER) of the megakaryocyte and subsequently packaged in secretory granules (eg, MMP2). These secretory granules are then transported through the proplatelet and delivered to platelets. Professional illustration by Marie Dauenheimer.