MATP is the home of the cream gene and its weaker cousin the pearl gene. Mutations in this gene cause tyrosinase (an important enzyme in making pigment) to be transported into the cell at much lower numbers. Without enough tyrosinase the cell can not make normal pigment amounts, making washed out pale looking horses. There is a much more noticeable effect on pheomelanin (red pigment) then eumelanin (black pigment) but the mutations do weaken production of both pigments. When heterozygous for the mutation (only having a single copy) horses become paler and more yellow in tone. Red/chestnut becomes palomino, bay becomes buckskin, blacks can be paler and more rust tinged etc. When homozygous (having two copies) pigment is greatly decreased and horses have a soft cream color through out. Eye color is also effected resulting in blue eyes in homozygous individuals.
Pearl occurred later in equine history so is not as common as cream. Heterozygous individuals undoubtedly have some pigment reduction but it is seldom noticeable. Homozygous pearl horses have pheomelanin and eumelanin both diluted, blue to green eyes, and a slight metallic sheen.
If a horse has one copy of pearl and one copy of cream a pseudo double cream occurs. The horses look like darker versions of double cream horses.
Official abbreviated name is MATP or SLC45A2
Proper full name is Membrane associated transport protein
Located on chromosome 21