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San Francisco-based retailer Gap Inc. disclosed a weaker-than-expected 6 percent drop in March same-store sales as Old Navy and its namesake brand stumbled at the end of the first quarter. Analysts had forecast a modest slide of 4.1 percent. Both Gap and Old Navy sales slumped 7 last month, while Banana Republic sales dipped 4 percent. Gap also reaffirmed its estimate for full-year earnings of $2.90 per share, compared with the Street’s view of $2.95. “While March performance has been challenging, we remain confident in the opportunities ahead,” chief executive Glenn Murphy said in a statement.