Cell transport network, conceptual artwork. Microtubules (blue and white strands) are protein fibres that act as a traffic network inside cells. The molecular cargo being transported along these 'roads' are represented by coloured spheres being carried by motor proteins (cartoon hands-and-feet figures). Helping to direct the 'traffic' are molecular tags, here shown as an array of traffic signs. These tags are formed by enzymes that modify the surface of the microtubules. The first of these enzymes were identified in 2005.