THE BRONX

Jealousy led one man to slash his ex-wife’s new boyfriend in the lobby of a Morrisania apartment building, police said.

Construction worker Issac Cassilla, 34, was leaving his lover’s fifth-floor apartment on the Grand Concourse near East 163rd Street when he ran into his 48-year-old assailant at 11 p.m. Friday, authorities said.

The ex-beau went ballistic and allegedly stabbed Cassilla in the neck, chest and right hand before fleeing.

Cassilla was rushed to Lincoln Hospital where he is listed in critical but stable condition, sources said. Cops are hunting for the ex.

A feud between two neighbors in a Melrose rooming house erupted in gunfire, leaving one of them dead, cops said yesterday.

James LeSane, 47, allegedly shot his 41-year-old upstairs neighbor, whose name was withheld, outside their Clay Avenue building near East 168th Street at 5 p.m. Friday.

The pair had been arguing earlier in the day and began fighting again, leading to the shooting.

The victim ran from the building and hid in the back seat of a parked car. LeSane allegedly jumped in the front seat and pumped a bullet into his chest.

The victim was rushed to Lincoln Hospital where he later died. LeSane was arrested at the scene.

QUEENS

A Rockaway Park man has been charged with breaking into his neighbor’s home, authorities said yesterday.

Brian Clancy, 27, kicked in the front door of a Beach 113th Street home at 6:45 p.m. Tuesday and was trying to get into a locked room on the second floor when the 43-year-old homeowner spotted him, authorities said.

Clancy, who was arrested Wednesday, is charged with burglary and criminal mischief, according to a spokeswoman for DA Richard Brown.

BROOKLYN

A teenager was shot on a Brownsville street corner yesterday, police said.

The 17-year-old victim, whose name was withheld, was rushed to Brookdale Hospital after he was shot in the chest at Strauss and Newport streets at 1:22 a.m., authorities said.

An off-duty city Department of Administrative Services employee has been charged with several offenses in Brownsville, police said yesterday.

Nicole Person, 30, was arrested at a social club on Putnam Avenue near Malcolm X Boulevard at 8:50 p.m. Friday, cops said.

She is charged with drug and weapon possession.

Erika Martinez,Tom

Liddy and John Doyle