A few days ago an article did the rounds on the regeneration of limbs in mammals being possible with the deleteion of a single gene. It's been known for some time that the humans have all the genes neccesary for limb regeneration, a set of genes very similar to lizards that can regenerate. The major difference being that when we lose a limb, the genes don't turn on.
A group from the Wistar Institute have published details of p21 knockout mice that appear to be able to regenerate limbs. p21 is a gene involved in the regulation of the cell cycle. There are various cyclin dependent kinases (CDK) floating around in a cell that help move a cell from a state of growth to a state of homeostasis and vice cersa. p21 binds to some of these CDK's and prevents the cell from moving into a growth phase. So by knocking out this gene in mice, the researchers appear to have been able to allow growth to move forward and the correct form of growth to progress to allow for limb regeneration.
The trouble with this is the p21 is part of the p53-tumour suppressor pathway. Basically, the p53 gene is turned on in response to DNA damage, which turns on p21 which in turn blocks the cell from replicating. Ideally, you do not want a cell to grow and replicate if it is damaged, so this a good thing. In fact, a damaged p53 gene is quite common in a variety of cancers, when the mechanism to stop damaged cells replicating is damaged, things go a little haywire and the damaged cells, can become cancerous.