The changes were sparked by observations that lymphopoiesis did not always break into two lineages at the level of the CLP. Worse, some macrophages (long considered a myeloid lineage) could be generated by lymphoid lineage progenitors. In essence focus has been shifted away from the CLP to the MLP[citation needed] (lymphoid-specified progenitors), which are clearly lymphoid progenitors yet retain some myeloid potential, particularly the ability in both humans and mice to make macrophages - one of the most versatile of immune cell defenders - and also many dendritic cells, the best 'watchdogs' of antigen invaders.