Also, researchers found a close relationship between the position of the first division of lateral root formation and the root tip. A cycB1:1::uidA selectable marker was used as a reporter for lateral root initiation and its early mitotic events. This marker was histochemically stained for beta-glucuronidase (GUS) in "Arabidopsis thalia" seedlings, which highlighted activity in the lateral root primordium and the transition zone between the hypocotyl and the root. Seedlings were harvested every day for a week and stained for GUS activity, then measured the primary root length as well as the distance to the root tip, the ratio between these two numbers being consistent. From this study, the following was concluded:   Auxin is responsible for generating concentration gradients to allow for proper plant development. As of 2020, one auxin transporter was identified as a means to flood the hormone into cells: AUXIN-RESISTANT1 (AUX1)/AUX1-LIKEs (LAXs). Also, two auxin transporters that allowed for the hormone to exit cells, PIN-FORMEDs (PINs) were established, as well as ATP-binding cassette Bs (ABCBs)/P-glycoproteins (PGPs). PIN proteins steer auxin to areas of necessity throughout the plant. These proteins present in the apical meristem of the plant direct auxin downward through the plant, a process independent of gravity. Once in the vicinity of the root, vascular cylinder cells shuttle auxin towards the center of the root cap. Lateral root cells then absorb the phytohormone through AUX1 permease. PIN proteins recirculate the auxin upwards to the plant shoots for direct access to the zone of elongation. Once utilized there, the proteins are then shuttled back to the lateral roots and their corresponding root caps. This entire process is known as the foundation model. In "Arabidopsis thaliana", PIN proteins are localized in cells based on the size of their loop that connects the intercellular matrix to the extracellular matrix. Shorter PIN proteins (PINs 1-4, 6, 7) are found intracellularly as well as nearest to the plasma membrane, whereas the longer proteins (PINs 5, 8) are found almost exclusively by the plasma membrane. The protein PIN8 significantly influences the development of lateral roots in a plant. When a nonfunctional mutant of the protein, "pin"8, was inserted into a plasmid, the lateral roots of "Arabidopsis thaliana" had a decrease in root density.