Regarding its biological functions, the relevance of Lck kinase for thymic development became clear very soon, as Lck deficiency generated a dramatic reduction in thymic cellularity associated with thymic atrophy, with a significant decrease in the CD4+CD8+ Double Positive (DP), and a decrease in CD4+ and CD8+ T cells in peripheral lymphoid organs [35]. Here, CD4 is linked to thymus atrophy.