M6P receptor proteins   Transmembrane receptor proteins present in the trans Golgi network that recognize the mannose 6-phosphate (M6P) groups added exclusively to lysosomal enzymes, marking the enzymes for packaging and delivery to early endosomes.
macromolecule Polymers constructed of long chains of covalently linked, small organic (carbon-containing) molecules. The principal building blocks from which a cell is constructed and the components that confer the most distinctive properties of living things.
macrophage   Phagocytic cell derived from blood monocytes, resident in most tissues but able to roam. It has both scavenger and antigen-presenting functions in immune responses.
macropinocytosis Clathrin-independent, dedicated degradative endocytic pathway induced in most cell types by cell-surface receptor activation by specific cargoes.
malaria   Protozoal disease caused by four species of Plasmodium, which are transmitted to humans by the bite of the female Anopheles mosquito.
malignant Of tumors and tumor cells: invasive and/or able to undergo metastasis. A malignant tumor is a cancer.  (Figure 20–3)
MAP kinase module (mitogen-activated protein kinase module) An intracellular signaling module composed of three protein kinases, acting in sequence, with MAP kinase as the third. Typically activated by a Ras protein in response to extracellular signals. (Figure 15–49) master transcription regulator   A transcription regulator specifically required for formation of a particular cell type. Artificial expression of master transcription regulators (alone or in combination with others) will often convert one cell type into another.
maternal inheritance A form of inheritance observed when following mitochondria in animals and plants, where mitochondrial DNA is inherited only through the female  germ line.
maternal-effect gene Gene that acts in the mother to specify maternal mRNAs and proteins in the egg. Maternal-effect mutations affect the development of the embryo even if the embryo itself has not inherited the mutated gene.
maternal-zygotic transition (MZT) Event in animal development where the embryo’s own genome largely takes over control of development from maternally deposited macromolecules.
proteolytic enzyme present in the extracellular matrix that degrades matrix proteins. Includes the collagenases.
matrix space   Large internal compartment of the mitochondrion.
mechanosensitive channels   Transmembrane ion channels that open in response to a mechanical stress on the lipid bilayer in which they are embedded.
megakaryocyte Large myeloid cell with a multilobed nucleus that remains in the bone marrow when mature. Buds off platelets from long cytoplasmic processes. (Figures 22–29) meiosis I   The first of two rounds of chromosome segregation following meiotic chromosome duplication; segregates the homologs, each composed of a tightly linked pair of sister chromatids.
meiosis II   The second of two rounds of chromosome segregation following meiotic chromosome duplication; segregates the sister chromatids of each homolog.