Resistance for the SPX is at the round-number 900 area, which is home to the heaviest out-of-the-money call open interest in the May series.

FORBES: Don't Sell This May

In fact, since the SPX burst above the round 1, 300 level on the first trading day of February, nearly 80% of the daily closes since that time have been between 1, 300 and 1, 333, and these remain support and resistance levels in the near term.

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Whereas double lows or previous resistance areas made themselves known to the SPX, RUT and DJIA, it was an important round-number level on the MID that came into play last week, as the index made its first-ever run at 1, 000 on Friday.

FORBES: Bullish Case Looks Tight At Key Technical Juncture