OnMouse...() Does not working.

What im doing wrong? No one of last functions dont workig (OnMouseDrag(), OnMouseDown(), etc.)

 using UnityEngine;
 using System.Collections;
 using UnityEngine.EventSystems;
 
 public class Letter : MonoBehaviour {
     bool mouse=false;
     public GameObject letter;
     void Start () {
         GameObject letterclone = (GameObject)Instantiate (letter,transform.position,transform.rotation);
         letterclone.transform.position = new Vector3(0,0,5);
         }
     void Update () {
 
             GameObject let = GameObject.Find("Letter(Clone)");
             Vector3 mousepos;
             mousepos = Input.mousePosition;
             mousepos.z = 5;
             mousepos.x -= 648;
             mousepos.y -= 330;
             let.transform.position = mousepos;
             //Debug.Log (mousepos);
             //Debug.Log ( let.transform.position);
     }
     void onMouseDown() {        
         Debug.Log ("On Mouse Down Event");
     }
     
     void OnMouseUp() {
         Debug.Log("On Mouse Up Event");
     }
     
     void OnMouseOver() {
         Debug.Log("On Mouse Over Event");    
     }
     
     void OnMouseEnter() {
         Debug.Log("On Mouse Enter Event");
     }
     
     void OnMouseDrag() {
         Debug.Log("On Mouse Drag Event");
     }
 }