Why would mutation of a tyrosine residue to glutamate in the intracellular portion of a growth factor receptor lead to increased tumor formation

this mutation leads to a constitutively active state where the receptor can not be inactivated by dephosphorylation  

Explanation:

Phosphorylation is a posttranslational modification that usually occurs at the side chains of serine, tyrosine and threonine residues, thereby regulating protein function. Phosphomimetic amino acids are substitutions (mutations) able to mimic the phosphorylated form of the protein, resulting in constitutive activation/inactivation of the corresponding protein. In this case, it is expected that the substitution of tyrosine to glutamate gives rise to a constitutively active state of the receptor by replacing tyrosine with a phosphomimetic residue.

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