Convert Transform to Gameobject

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {

public GameObject a;
public Transform b;

void OnMouseEnter(){
b=a;
}

Error is :
error CS0029: Cannot implicitly convert type UnityEngine.GameObject' to UnityEngine.Transform’

what can i do?

Every GameObject has a Transform and every Transform has a GameObject. The Easiest thing to do would be to have:

void OnMouseEnter(){
b=a.transform;
}