Joseph, Philip and Paul Castellano Jr. simply went on with their lives after then-mob upstart John Gotti killed their dad in 1986. Their actions suggest that they understood that the killing was, as Don Vito Corleone noted, "only business."

Not to William Cutolo Jr. (right). He adored his father and the father doted on his children and grandkids.

Within days of his old man's disappearance in the spring of 1999, young Cutolo began helping the FBI and Brooklyn federal prosecutors in their quest to nail acting family boss Alphonse Persico and underboss John (Jackie) DeRoss for Wild Bill's slaying.

The feds haven't yet charged anyone with the elder Cutolo's murder, but young Cutolo has helped the feds nail the main suspects, Persico, 46, and DeRoss, 63, on racketeering charges.

While working with the feds, Cutolo Jr. tipped them about loansharking records that Persico maintained in a Brooklyn apartment, told them that DeRoss had taken over his father's crew, and wore a wire against them, law enforcement sources said.

He will testify at their upcoming trial, assistant U.S. attorneys Amy Walsh and John Kroger announced this week at the arraignment of two codefendants

Sources said that Cutolo's widow and other family members have supported William Jr.'s decision and have been relocated under the federal Witness Protection Program.

"Wild Bill must be turning over in his grave," said one underworld source.

"I think he's smiling, knowing his kid is getting even, and will live to enjoy it," a law enforcement source disagreed.

According to an affidavit by FBI agent Gary Pontecorvo, young Cutolo, 30, identified as "CW"for cooperating witness, voluntarily began providing information about his own criminal activities and those of Colombo mobsters and associates in June, 1999, just days after his father's disappearance on May 26.

Sources said he admitted making loanshark collections for his father when he was jailed and awaiting trial for racketeering and murder conspiracy

Last March, said Pontecorvo, Cutolo Jr. reported that Persico, (left) then serving 15 months for weapons possession, funneled orders to him through DeRoss, who had taken over Wild Bill's crew when he disappeared. In June, he reported that DeRoss (right) said he had been promoted to family underboss.

"CW has proved reliable and trustworthy in all respects, except one, to wit: (he) committed a crime while he was cooperating," said Pontecorvo, referring to Cutolo Jr.'s extortion of $15,000 from a stock broker last year.

The charges were dismissed on Nov. 3, according to court records, but since then, said Pontecorvo, young Cutolo pleaded guilty to criminal charges in that incident as part of a cooperation agreement with the feds that is still sealed.

Lawyers James LaRossa and Andrew Weinstein, who represented Cutolo Jr. in the extortion case, now represent Dionisio and Locascio, respectively. The prosecutors officially notified the defendants and lawyers of Cutolo Jr.'s cooperation to enable all parties to determine whether the defendants should retain new counsel with no apparent conflicts of interest.

Brooklyn Federal Judge Reena Raggi told the defendants and lawyers to determine within 10 days whether or not Dionisio and Locascio would be better served by attorneys without links to Cutolo Jr.

How about the letter "J" that appears in his first name and "Joe P"at the left?

The feds -- who say the list of names are loansharking records -- believe that DeRoss penned the list and it ties both Persico and DeRoss to a loansharking business involving members of Wild Bill Cutolo's former crew who are codefendants in the case. Dom refers to Dionisio, Mike D to Michael Donato, Joe P to Joseph Petillo and Ralph to Ralph Guccione.

"The government expects a handwriting expert to testify at trial that these loansharking records were written by DeRoss," Walsh and Kroger said in court papers. "Even a non-expert" could see that "the upper-case "D" is made with a large loop on the lower left side in both the records and the exemplars," they assert.

Citing the handwriting evidence and allegations that DeRoss, who was convicted of racketeering in 1986, violated his parole in 1993, 1994, and 1996 and would engage in mob business if released on bail, the prosecutors sought to detain DeRoss.

Brooklyn Federal Judge Reena Raggi, who detained Persico last month, came down on the side of DeRoss and defense lawyer Paul LeMole, setting bail at $4million and ordering DeRoss confined to his Staten Island home under strict house arrest conditions.