NEW YORK (AP) — Stocks that traded heavily or had substantial price changes on Monday:

AT&T Inc., down 10 cents to $17.50.

The telecommunications giant said it discovered a data breach impacting millions of current and former customers.

Elanco Animal Health Inc., down 31 cents to $15.97.

The company reportedly reached a settlement with investment firm Ancora.

Appian Corp., down $2.13 to $37.82.

The business process management software provider said its chief revenue officer resigned.

Ballard Power Systems Inc., up 49 cents to $3.27.

The fuel cell technology company announced a supply agreement with Solaris Bus & Coach.

Semtech Corp., up $1.88 to $29.37.

The chipmaker gave investors an encouraging revenue forecast.

Guess Inc., up $1.93 to $33.40.

The clothing company announced a $200 million stock buyback plan.

Baxter International Inc., up 81 cents to $43.55.

The drug and medical device maker received regulatory clearance in the U.S. for an infusion pump.

FedEx Corp., down $9.61 to $280.13.

The package delivery service said that its contract with the U.S. Postal Service expires in September.